For with what confidence can I venture upon such bold enterprises, when beside those numberless infirmities peculiar to myself, I find so many which are common to human nature? Can I be sure, that in leaving all established opinions I am following truth; and by what criterion shall I distinguish her, even if fortune should at last guide me on her foot-steps? After the most accurate and exact of my reasonings, I can give no reason why I should assent to it; and feel nothing but a strong propensity to consider objects strongly in that view, under which they appear to me. Experience is a principle, which instructs me in the several conjunctions of objects for the past. Habit is another principle, which determines me to expect the same for the future; and both of them conspiring to operate upon the imagination, make me form certain ideas in a more intense and lively manner, than others, which are not attended with the same advantages. Without this quality, by which the mind enlivens some ideas beyond others (which seemingly is so trivial, and so little founded on reason) we could never assent to any argument, nor carry our view beyond those few objects, which are present to our senses. Nay, even to these objects we could never attribute any existence, but what was dependent on the senses; and must comprehend them entirely in that succession of perceptions, which constitutes our self or person. Nay farther, even with relation to that succession, we could only admit of those perceptions, which are immediately present to our consciousness, nor could those lively images, with which the memory presents us, be ever received as true pictures of past perceptions. The memory, senses, and understanding are, therefore, all of them founded on the imagination, or the vivacity of our ideas. No wonder a principle so inconstant and fallacious should lead us into errors, when implicitly followed (as it must be) in all its variations. It is this principle, which makes us reason from causes and effects; and it is the same principle, which convinces us of the continued existence of external objects, when absent from the senses. But though these two operations be equally natural and necessary in the human mind, yet in some circumstances they are[16] directly contrary, nor is it possible for us to reason justly and regularly from causes and effects, and at the same time believe the continued existence of matter. How then shall we adjust those principles together? Which of them shall we prefer? Or in case we prefer neither of them, but successively assent to both, as is usual among philosophers, with what confidence can we afterwards usurp that glorious title, when we thus knowingly embrace a manifest contradiction? [16] Sect. 4.