Since all impressions are internal and perishing existences, and appear as such, the notion of their distinct and continued existence must arise from a concurrence of some of their qualities with the qualities of the imagination, and since this notion does not extend to all of them, it must arise from certain qualities peculiar to some impressions.  It will therefore be easy for us to discover these qualities by a comparison of the impressions, to which we attribute a distinct and continued existence, with those, which we regard as internal and perishing.

We may observe, then, that it is neither upon account of the involuntariness of certain impressions, as is commonly supposed, nor of their superior force and violence, that we attribute to them a reality, and continued existence, which we refuse to others, that are voluntary or feeble.  For it is evident our pains and pleasures, our passions and affections, which we never suppose to have any existence beyond our perception, operate with greater violence, and are equally involuntary, as the impressions of figure and extension, colour and sound, which we suppose to be permanent beings.  The heat of a fire, when moderate, is supposed to exist in the fire; but the pain, which it causes upon a near approach, is not taken to have any being, except in the perception.

These vulgar opinions, then, being rejected, we must search for some other hypothesis, by which we may discover those peculiar qualities in our impressions, which makes us attribute to them a distinct and continued existence.

After a little examination, we shall find, that all those objects, to which we attribute a continued existence, have a peculiar constancy, which distinguishes them from the impressions, whose existence depends upon our perception.  Those mountains, and houses, and trees, which lie at present under my eye, have always appeared to me in the same order; and when I lose sight of them by shutting my eyes or turning my head, I soon after find them return upon me without the least alteration.  My bed and table, my books and papers, present themselves in the same uniform manner, and change not upon account of any interruption in my seeing or perceivilng them.  This is the case with all the impressions, whose objects are supposed to have an external existence; and is the case with no other impressions, whether gentle or violent, voluntary or involuntary.

This constancy, however, is not so perfect as not to admit of very considerable exceptions.  Bodies often change their position and qualities, and after a little absence or interruption may become hardly knowable.  But here it is observable, that even in these changes they preserve a coherence, and have a regular dependence on each other; which is the foundation of a kind of reasoning from causation, and produces the opinion of their continued existence.  When I return to my chamber after an hour’s absence, I find not my fire in the same situation, in which I left it:  But then I am accustomed in other instances to see a like alteration produced in a like time, whether I am present or absent, near or remote.  This coherence, therefore, in their changes is one of the characteristics of external objects, as well as their constancy.