Aphorism First, Corollary.

We know our existence only by the impressions we experience, and that of beings other than ourselves, but by the impressions which they cause on us.

In like manner, as all our propositions may be reduced to the form of enunciative propositions, because at bottom they all express a judgment, so all our enunciative propositions may afterwards be always reduced to some one of these: I think, I feel, or I perceive, that such a thing is in such a manner, or that such a being produces such an effect; propositions of which we are ourselves the subject, because in fact we are always the subject of all our judgments, since they never express but the impression which we experience.

From hence it follows: 1st. That our perceptions are all of them always such as we feel them, and are not susceptible of any error, taken each separately, and in itself.

2dly. That if in the different combinations, we make of them, we add to them nothing which is not primitively comprised in them, implicitly or explicitly, they are always conformable to the existence of the beings which cause them, since that existence is not known to us but by them, and consists for us only in those perceptions.

3dly. That we know nothing but relatively to ourselves, and to our means of receiving perceptions.

4thly. That these perceptions are every thing for us; that we know nothing ever but our perceptions; that they are the only things truly real for us, and that the reality which we recognize in the beings that cause them is only secondary, and consists only in the permanent power of always causing the same impressions under the same circumstances, whether on ourselves, or on other sensible beings, who give us an account of them (also by the impressions which they cause in us) when we have become able to hold communication with them by signs.