When we become aware of the two sensations to be compared, their impression is made, each object is perceived, both are perceived, but for all that their relation is not perceived.  If the judgment of this relation were merely a sensation, and came to me solely from the object itself, my judgments would never be mistaken, for it is never untrue that I feel what I feel.

Why then am I mistaken as to the relation between these two sticks, especially when they are not parallel?  Why, for example, do I say the small stick is a third of the large, when it is only a quarter?  Why is the picture, which is the sensation, unlike its model which is the object?  It is because I am active when I judge, because the operation of comparison is at fault; because my understanding, which judges of relations, mingles its errors with the truth of sensations, which only reveal to me things.

Add to this a consideration which will, I feel sure, appeal to you when you have thought about it:  it is this—­If we were purely passive in the use of our senses, there would be no communication between them; it would be impossible to know that the body we are touching and the thing we are looking at is the same.  Either we should never perceive anything outside ourselves, or there would be for us five substances perceptible by the senses, whose identity we should have no means of perceiving.

This power of my mind which brings my sensations together and compares them may be called by any name; let it be called attention, meditation, reflection, or what you will; it is still true that it is in me and not in things, that it is I alone who produce it, though I only produce it when I receive an impression from things.  Though I am compelled to feel or not to feel, I am free to examine more or less what I feel.

Being now, so to speak, sure of myself, I begin to look at things outside myself, and I behold myself with a sort of shudder flung at random into this vast universe, plunged as it were into the vast number of entities, knowing nothing of what they are in themselves or in relation to me.  I study them, I observe them; and the first object which suggests itself for comparison with them is myself.

All that I perceive through the senses is matter, and I deduce all the essential properties of matter from the sensible qualities which make me perceive it, qualities which are inseparable from it.  I see it sometimes in motion, sometimes at rest, [Footnote:  This repose is, if you prefer it, merely relative; but as we perceive more or less of motion, we may plainly conceive one of two extremes, which is rest; and we conceive it so clearly that we are even disposed to take for absolute rest what is only relative.  But it is not true that motion is of the essence of matter, if matter may be conceived of as at rest.] hence I infer that neither motion nor rest is essential to it, but motion, being an action, is the result of a cause of which rest is only the absence.  When, therefore, there is nothing acting upon matter it does not move, and for the very reason that rest and motion are indifferent to it, its natural state is a state of rest.