George Berkeley
An Essay Towards a New Theory of Vision (4th ed.)
George Berkeley (1732)
First edition published 1709
1. My design is to shew the manner wherein we perceive by sight the distance,
magnitude, and situation of objects. Also to consider the difference there is
betwixt the ideas of sight and touch, and whether there be any idea common to
both senses.
2. It is, I think, agreed by all that distance, of itself and immediately,
cannot be seen. For distance being a Line directed end-wise to the eye, it
projects only one point in the fund of the eye, which point remains invariably
the same, whether the distance be longer or shorter.
3. I find it also acknowledged that the estimate we make of the distance of
objects considerably remote is rather an act of judgment grounded on experience
than of sense. For example, when I perceive a great number of intermediate
objects, such as houses, fields, rivers, and the like, which I have experienced
to take up a considerable space, I thence form a judgment or conclusion that the
object I see beyond them is at a great distance. Again, when an object appears
faint and small, which at a near distance I have experienced to make a vigorous
and large appearance, I instantly conclude it to be far off: And this, 'tis
evident, is the result of experience; without which, from the faintness and
littleness I should not have inferred anything concerning the distance of
objects.
4. But when an object is placed at so near a distance as that the interval
between the eyes bears any sensible proportion to it, the opinion of speculative
men is that the two optic axes (the fancy that we see only with one eye at once
being exploded) concurring at the object do there make an angle, by means of
which, according as it is greater or lesser, the object is perceived to be
nearer or farther off.[1]
5. Betwixt which and the foregoing manner of estimating distance there is this
remarkable difference: that whereas there was no apparent, necessary connexion
between small distance and a large and strong appearance, or between great
distance and a little and faint appearance, there appears a very necessary
connexion between an obtuse angle and near distance, and an acute angle and
farther distance. It does not in the least depend upon experience, but may be
evidently known by anyone before he had experienced it, that the nearer the
concurrence of the optic axes, the greater the angle, and the remoter their
concurrence is, the lesser will be the angle comprehended by them.
6. There is another way mentioned by optic writers, whereby they will have us
judge of those distances, in respect of which the breadth of the pupil hath any
sensible bigness: And that is the greater or lesser divergency of the rays,
which issuing from the visible point do fall on the pupil, that point being
judged nearest which is seen by most diverging rays, and that remoter which is
seen by less diverging rays: and so on, the apparent distance still increasing,
as the divergency of the rays decreases, till at length it becomes infinite,
when the rays that fall on the pupil are to sense parallel. And after this
manner it is said we perceive distance when we look only with one eye.
7. In this case also it is plain we are not beholding to experience: it being a
certain, necessary truth that the nearer the direct rays falling on the eye
approach to a parallelism, the farther off is the point of their intersection,
or the visible point from whence they flow.
8. Now though the accounts here given of perceiving near distance by sight are
received for true, and accordingly made use of in determining the apparent
places of objects, they do nevertheless seem very unsatisfactory: and that for
these following reasons.
9. It is evident that when the mind perceives any idea, not immediately and of
itself, it must be by the means of some other idea. Thus, for instance, the
passions which are in the mind of another are of themselves to me invisible. I
may nevertheless perceive them by sight, though not immediately, yet by means of
the colours they produce in the countenance. We often see shame or fear in the
looks of a man, by perceiving the changes of his countenance to red or pale.
10. Moreover it is evident that no idea which is not itself perceived can be the
means of perceiving any other idea. If I do not perceive the redness or paleness
of a man's face themselves, it is impossible I should perceive by them the
passions which are in his mind.
11. Now from sect. 2 it is plain that distance is in its own nature
imperceptible, and yet it is perceived by sight. It remains, therefore, that it
be brought into view by means of some other idea that is itself immediately
perceived in the act of vision.
I2. But those lines and angles, by means whereof some men pretend to explain the
perception of distance, are themselves not at all perceived, nor are they in
truth ever thought of by those unskilful in optics. I appeal to anyone's
experience whether upon sight of an object he computes its distance by the
bigness of the angle made by the meeting of the two optic axes? Or whether he
ever thinks of the greater or lesser divergency of the rays, which arrive from
any point to his pupil? Everyone is himself the best judge of what he perceives,
and what not. in vain shall any man tell me various ideas of distance, so long
as I myself am conscious of no such thing.
13. Since, therefore, those angles and lines are not themselves perceived by
sight, it follows from sect. 10 that the mind doth not by them judge of the
distance of objects.
14. The truth of this assertion will be yet farther evident to anyone that
considers those lines and angles have no real existence in nature, being only an
hypothesis framed by the mathematicians, and by them introduced into optics,
that they might treat of that science in a geometrical way.
15. The last reason I shall give for rejecting that doctrine is, that though we
should grant the real existence of those optic angles, etc., and that it was
possible for the mind to perceive them, yet these principles would not be found
sufficient to explain the phenomena of distance, as shall be shewn hereafter.
16. Now, it being already shewn that distance is suggested to the mind by the
mediation of some other idea which is itself perceived in the act of seeing, it
remains that we inquire what ideas or sensations there be that attend vision,
unto which we may suppose the ideas of distance are connected, and by which they
are introduced into the mind. And first, it is certain by experience that when
we look at a near object with both eyes, according as it approaches or recedes
from us, we alter the disposition of our eyes, by lessening or widening the
interval between the pupils. This disposition or turn of the eyes is attended
with a sensation, which seems to me to be that which in this case brings the
idea of greater or lesser distance into the mind.
17. Not that there is any natural or necessary connexion between the sensation
we perceive by the turn of the eyes and greater or lesser distance, but because
the mind has by constant experience found the different sensations corresponding
to the different dispositions of the eyes to be attended each with a different
degree of distance in the object, there has grown an habitual or customary
connexion between those two sorts of ideas, so that the mind no sooner perceives
the sensation arising from the different turn it gives the eyes, In order to
bring the pupils nearer or farther asunder, but it withal perceives the
different idea of distance which was wont to be connected with that sensation;
just as upon hearing a certain sound, the idea is immediately suggested to the
understanding which custom had united with it.
18 Nor do I see how I can easily be mistaken in this matter. I know evidently
that distance is not perceived of itself. That by consequence it must be
perceived by means of some other idea which is immediately perceived, and varies
with the different degrees of distance. I know also that the sensation arising
from the turn of the eyes is of itself immediately perceived, and various
degrees thereof are connected with different distances, which never fail to
accompany them into my mind, when I view an object distinctly with both eyes,
whose distance is so small that in respect of it the interval between the eyes
has any considerable magnitude.
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