A single perception can never produce the idea of a double existence, but by some inference either of the reason or imagination.

When the mind looks farther than what immediately appears to it, its conclusions can never be put to the account of the senses; and it certainly looks farther, when from a single perception it infers a double existence, and supposes the relations of resemblance and causation betwixt them.
If our senses, therefore, suggest any idea of distinct existences, they must convey the impressions as those very existences, by a kind of fallacy and illusion.

Upon this bead we may observe, that all sensations are felt by the mind, such as they really are, and that when we doubt, whether they present themselves as distinct objects, or as mere impressions, the difficulty is not concerning their nature, but concerning their relations and situation.

Now if the senses presented our impressions as external to, and independent of ourselves, both the objects and ourselves must be obvious to our senses, otherwise they coued not be compared by these faculties.

The difficulty, then, is how fax we are ourselves the objects of our senses.
It is certain there is no question in philosophy more abstruse than that concerning identity, and the nature of the uniting principle, which constitutes a person.