" INFALLIBILITY" OF PERCEPTION.

 Now from such facts a few

inferences are fairly obvious. The^sY is that the sense quality

with which the datum or object presents itself to the conscious

ness of the perceiver is not exclusively external, in the sense of

belonging to the external reality altogether independently of the

actual condition of the sense organ through which the datum is

presented.

A second is that we must carefully distinguish between the

actually presented datum or object itself (or its mere presentation

to the perceiver), and the spontaneous judgment whereby the

latter interprets it (or represents it to himself as being such or

such).

1 Cf. MAHER, op. cit., pp. 171-8 ; JEANNIERE, op. cit., p. 388 n.

92 THEOR y OF KNO WLEDGE

A third is that not only is intellect thus spontaneously co

operating with external sense, but that also the imagination

with its stock of images acquired in past perceptions and stored

up through memory and mental association is constantly sup

plementing, subjectively moulding and filling in, the presented

and perceived external datum or fraction of external reality : so

that the spontaneous intellectual interpretation is never of the

bare percept itself but rather of the percept in its whole concrete,

sensuous, psychic context.

^fourth is that since the actual organic condition of the sen

tient subject, i.e. of the perceiver s brain, nervous system, sense

organs, is a partial determinant of the concretely "qualified"

datum or presented object, if this condition be abnormal it may

cause a datum or object to be presented as external which has

no reality at all outside the perceiver s own organism ; or whose

reality outside and independently of the perceiver s organism is

presented to the perceiver (owing to the abnormal and disturb

ing co-operation of the latter s organism) otherwise than it would

be presented to the normal perceiver : in which case the spon

taneous judgment of the abnormal perceiver will make him the

victim of an hallucination or an illusion until such time as intel

lectual advertence to his own abnormal condition will enable him

to rectify this judgment. And this is true whether the abnormal

organic condition be congenital or supervening, permanent or

transitory, curable or incurable.

A fifth inference is that, distinguishing between the whole

concrete datum or object presented to consciousness by the co

operation of (a) the external reality, (b) the external sense organ

(with the brain and nervous system) subserving perception, and

(c) the brain and nervous system subserving the internal senses

(the faculties of association, imagination, and memory), distin

guishing between this whole concrete datum on the one hand,

and the perceiver s spontaneous interpretation of it on the other,

the former is in every case necessarily what it is and as it is,

and the latter alone can be erroneous. In the concrete condi

tions, objective and subjective, whether normal or abnormal, in

which the datum is presented to consciousness, that datum could

not be other than it is. The man plunging his heated hand into

lukewarm water does really feel a "concrete cold" datum, and

plunging his cold hand into the same water hC does really feel

a " concrete hot " datum. The person to whose diseased palate

RELATIVITY OF SENSE QUALITIES TO PERCEIVER 93

sugar tastes bitter is really aware of the concrete sense datum,

" bitter ". The colour-blind perceiver, looking on the field of

poppies has really present to consciousness a datum which he

rightly designates " grey " or " green ". The person who presses

the corner of one of his eyes is really aware of a twofold datum

or object. In dreams, hallucinations and illusions he really has

presented in consciousness the data or objects which he thinks

to be external things, or to be such or such external things. In

other words, the senses themselves neither err nor deceive. They

do not err because they do not judge or interpret, but merely

present, register, report a " something," a " datum," an " object "

to the conscious perceiver. They do not themselves deceive be

cause they always present or register or report that precisely

which under the circumstances they must : they simply could

not present a datum other or otherwise than they actually do :

according to the organic condition in which they are, and accord

ing to the condition in which the external influence impresses

them, so must the presented datum be, nor can it be otherwise :

nor can the perceiver be deceived in judging that he has this

datum consciously present to him. 1

But the presentation of a sense datum can be an occasion of

deception to the perceiver, inasmuch as the latter may judge that

the datum is external, or how it is externally, without adverting

to the fact that the presented datum is partially determined by

the condition of his own organism, and that this condition is,

perhaps, abnormal. And just as the subjective, organic condi

tion of the perceiver may be an occasion of error in his spon

taneous judgment, so may the abnormal condition of the external

thing itself, or of the physical medium spatially intervening

between the latter and the sense organ of the perceiver." A trite

and telling example of this source of error is the familiar fact

that a straight stick partially immersed in water and seen ob

liquely appears bent. Or again, to a person sitting in a moving

1 As St. Thomas expresses it (Sitmma TheoL, I., Q. xvii., a. 2) : " Per hoc quod

sensus ita nuntiant sicut afficiuntur, sequitur quod non decipiamur in judicio quo

judicamus nos sentire aliquid ; sed ex eo quod sensus aliter afficitur interdttm quam

res sit, sequitur quod nuntiet nobis aliquando rem aliter quam sit, et ex hoc fallimur

per sensum circa rent, non circa ipsum sentire". (Italics ours: "aliter afficitur

. . . quam res sit," i.e. otherwise than the normal sense is affected by the thing in

normal external conditions : for it is the normal sense, perceiving the external thing

in normal external conditions, that enables us to discover how the thing really is,

" quomodo res sit ").

2 Cf. preceding note.

94 THE OR Y OF KNO l\ LED CE

train which is passing another train that is stationary, the latter

appears to be moving and the former at rest : a double or com

pound illusion. Or, an object seen through the microscope ap

pears much larger than "it really is". Or, two plane images

placed side by side and seen through the stereoscope appear as

one object in relief. Or, we still see in the heavens stars which

ages ago ceased to emit light. Or, the setting sun appears as

visible above the horizon when it is really below the horizon.

Or, certain atmospheric conditions at sea or in the desert pro

duce the optical illusion, known as the mirage, of ships, trees,

etc., seen inverted in the heavens.

Now such " illusions of the senses," though puzzling to the

plain man, have never shaken his spontaneous belief in the trust

worthiness of his senses under normal conditions. But philoso

phers, who have tried to think out the bearing of these illusions

on our spontaneous beliefs regarding the existence, qualities, and

nature of an external domain of reality, have been more than

puzzled by such illusions : many have been driven by them into

the position of theoretical scepticism, subjectivism or idealism. 1

This, however, is an unjustifiable conclusion if it can be shown,

as we hope to show, that such errors and illusions can be both

explained and corrected by reflection.

For the realist, however, who holds that sense data reveal to

us not only the existence, but in some measure the qualities and

nature, of an external domain of reality, they do raise a serious

question as to whether or how far this external domain of real

ity has, apart from sense perception, the ("secondary" and

"primary") sense qualities of which we become aware in the

actual process of perception. Following up, therefore, in the

light of those " illusions," the inferences set forth above, infer

ences which already partly explain the illusions, we may ask

this general question: If the presented "external" sense data

partly depend, as regards the concrete qualities with which they

present themselves to the conscious perceiver, on organic con

ditions of the perceiving self or subject, can we determine whether

or how far those qualities are really in the external domain of

reality independently of the perceiving self or subject? Well,

if we include the perceiver s own organism in the domain of

material reality which has been proved to exist independently

of actual perception, independently of the perceiver s conscious-

1 Cf. infra, 128.

RELATIVITY OF SENSE QUALITIES TO PERCEIVER 95

ness, and which is in this sense extramental as opposed to what

is essentially dependent on mind or consciousness, there seems

to be no valid reason for denying or doubting that those sense

qualities, secondary and primary, are qualities of at any rate

extramental or material reality. To this we shall return presently

for the purpose of examining the distinction drawn between the

potential and the actual reality which some of those qualities are

supposed to have in the material domain, apart from perception,

and in perception, respectively.

If, however, we ask whether or how we are justified in locat

ing those qualities in the external domain of material things,

apart from and independently of the role played in perception

by the material sense organs of the perceiver s body, we shall find

it necessary to recognize explicitly the import of a distinction

which in practice we are always making implicitly, spontaneously,

and unreflectingly, in our ordinary processes of external sense

perception : namely the distinction between concrete, " qualified "

data or objects apprehended by the perceiver in normal conditions,

organic or subjective or internal, and physical or objective or

external, and other such data perceived in abnormal conditions.

 Now from such facts a few

inferences are fairly obvious. The^sY is that the sense quality

with which the datum or object presents itself to the conscious

ness of the perceiver is not exclusively external, in the sense of

belonging to the external reality altogether independently of the

actual condition of the sense organ through which the datum is

presented.

A second is that we must carefully distinguish between the

actually presented datum or object itself (or its mere presentation

to the perceiver), and the spontaneous judgment whereby the

latter interprets it (or represents it to himself as being such or

such).

1 Cf. MAHER, op. cit., pp. 171-8 ; JEANNIERE, op. cit., p. 388 n.

92 THEOR y OF KNO WLEDGE

A third is that not only is intellect thus spontaneously co

operating with external sense, but that also the imagination

with its stock of images acquired in past perceptions and stored

up through memory and mental association is constantly sup

plementing, subjectively moulding and filling in, the presented

and perceived external datum or fraction of external reality : so

that the spontaneous intellectual interpretation is never of the

bare percept itself but rather of the percept in its whole concrete,

sensuous, psychic context.

^fourth is that since the actual organic condition of the sen

tient subject, i.e. of the perceiver s brain, nervous system, sense

organs, is a partial determinant of the concretely "qualified"

datum or presented object, if this condition be abnormal it may

cause a datum or object to be presented as external which has

no reality at all outside the perceiver s own organism ; or whose

reality outside and independently of the perceiver s organism is

presented to the perceiver (owing to the abnormal and disturb

ing co-operation of the latter s organism) otherwise than it would

be presented to the normal perceiver : in which case the spon

taneous judgment of the abnormal perceiver will make him the

victim of an hallucination or an illusion until such time as intel

lectual advertence to his own abnormal condition will enable him

to rectify this judgment. And this is true whether the abnormal

organic condition be congenital or supervening, permanent or

transitory, curable or incurable.

A fifth inference is that, distinguishing between the whole

concrete datum or object presented to consciousness by the co

operation of (a) the external reality, (b) the external sense organ

(with the brain and nervous system) subserving perception, and

(c) the brain and nervous system subserving the internal senses

(the faculties of association, imagination, and memory), distin

guishing between this whole concrete datum on the one hand,

and the perceiver s spontaneous interpretation of it on the other,

the former is in every case necessarily what it is and as it is,

and the latter alone can be erroneous. In the concrete condi

tions, objective and subjective, whether normal or abnormal, in

which the datum is presented to consciousness, that datum could

not be other than it is. The man plunging his heated hand into

lukewarm water does really feel a "concrete cold" datum, and

plunging his cold hand into the same water hC does really feel

a " concrete hot " datum. The person to whose diseased palate

RELATIVITY OF SENSE QUALITIES TO PERCEIVER 93

sugar tastes bitter is really aware of the concrete sense datum,

" bitter ". The colour-blind perceiver, looking on the field of

poppies has really present to consciousness a datum which he

rightly designates " grey " or " green ". The person who presses

the corner of one of his eyes is really aware of a twofold datum

or object. In dreams, hallucinations and illusions he really has

presented in consciousness the data or objects which he thinks

to be external things, or to be such or such external things. In

other words, the senses themselves neither err nor deceive. They

do not err because they do not judge or interpret, but merely

present, register, report a " something," a " datum," an " object "

to the conscious perceiver. They do not themselves deceive be

cause they always present or register or report that precisely

which under the circumstances they must : they simply could

not present a datum other or otherwise than they actually do :

according to the organic condition in which they are, and accord

ing to the condition in which the external influence impresses

them, so must the presented datum be, nor can it be otherwise :

nor can the perceiver be deceived in judging that he has this

datum consciously present to him. 1

But the presentation of a sense datum can be an occasion of

deception to the perceiver, inasmuch as the latter may judge that

the datum is external, or how it is externally, without adverting

to the fact that the presented datum is partially determined by

the condition of his own organism, and that this condition is,

perhaps, abnormal. And just as the subjective, organic condi

tion of the perceiver may be an occasion of error in his spon

taneous judgment, so may the abnormal condition of the external

thing itself, or of the physical medium spatially intervening

between the latter and the sense organ of the perceiver." A trite

and telling example of this source of error is the familiar fact

that a straight stick partially immersed in water and seen ob

liquely appears bent. Or again, to a person sitting in a moving

1 As St. Thomas expresses it (Sitmma TheoL, I., Q. xvii., a. 2) : " Per hoc quod

sensus ita nuntiant sicut afficiuntur, sequitur quod non decipiamur in judicio quo

judicamus nos sentire aliquid ; sed ex eo quod sensus aliter afficitur interdttm quam

res sit, sequitur quod nuntiet nobis aliquando rem aliter quam sit, et ex hoc fallimur

per sensum circa rent, non circa ipsum sentire". (Italics ours: "aliter afficitur

. . . quam res sit," i.e. otherwise than the normal sense is affected by the thing in

normal external conditions : for it is the normal sense, perceiving the external thing

in normal external conditions, that enables us to discover how the thing really is,

" quomodo res sit ").

2 Cf. preceding note.

94 THE OR Y OF KNO l\ LED CE

train which is passing another train that is stationary, the latter

appears to be moving and the former at rest : a double or com

pound illusion. Or, an object seen through the microscope ap

pears much larger than "it really is". Or, two plane images

placed side by side and seen through the stereoscope appear as

one object in relief. Or, we still see in the heavens stars which

ages ago ceased to emit light. Or, the setting sun appears as

visible above the horizon when it is really below the horizon.

Or, certain atmospheric conditions at sea or in the desert pro

duce the optical illusion, known as the mirage, of ships, trees,

etc., seen inverted in the heavens.

Now such " illusions of the senses," though puzzling to the

plain man, have never shaken his spontaneous belief in the trust

worthiness of his senses under normal conditions. But philoso

phers, who have tried to think out the bearing of these illusions

on our spontaneous beliefs regarding the existence, qualities, and

nature of an external domain of reality, have been more than

puzzled by such illusions : many have been driven by them into

the position of theoretical scepticism, subjectivism or idealism. 1

This, however, is an unjustifiable conclusion if it can be shown,

as we hope to show, that such errors and illusions can be both

explained and corrected by reflection.

For the realist, however, who holds that sense data reveal to

us not only the existence, but in some measure the qualities and

nature, of an external domain of reality, they do raise a serious

question as to whether or how far this external domain of real

ity has, apart from sense perception, the ("secondary" and

"primary") sense qualities of which we become aware in the

actual process of perception. Following up, therefore, in the

light of those " illusions," the inferences set forth above, infer

ences which already partly explain the illusions, we may ask

this general question: If the presented "external" sense data

partly depend, as regards the concrete qualities with which they

present themselves to the conscious perceiver, on organic con

ditions of the perceiving self or subject, can we determine whether

or how far those qualities are really in the external domain of

reality independently of the perceiving self or subject? Well,

if we include the perceiver s own organism in the domain of

material reality which has been proved to exist independently

of actual perception, independently of the perceiver s conscious-

1 Cf. infra, 128.

RELATIVITY OF SENSE QUALITIES TO PERCEIVER 95

ness, and which is in this sense extramental as opposed to what

is essentially dependent on mind or consciousness, there seems

to be no valid reason for denying or doubting that those sense

qualities, secondary and primary, are qualities of at any rate

extramental or material reality. To this we shall return presently

for the purpose of examining the distinction drawn between the

potential and the actual reality which some of those qualities are

supposed to have in the material domain, apart from perception,

and in perception, respectively.

If, however, we ask whether or how we are justified in locat

ing those qualities in the external domain of material things,

apart from and independently of the role played in perception

by the material sense organs of the perceiver s body, we shall find

it necessary to recognize explicitly the import of a distinction

which in practice we are always making implicitly, spontaneously,

and unreflectingly, in our ordinary processes of external sense

perception : namely the distinction between concrete, " qualified "

data or objects apprehended by the perceiver in normal conditions,

organic or subjective or internal, and physical or objective or

external, and other such data perceived in abnormal conditions.