First principles must, accordingly, be of two kinds. Of those that are based upon experience, I will give the following instances:--I hear the chirping of a bird, and I see an inkstand before me. That I have the sensation of hearing and seeing in these two cases, are facts of which it is impossible I can doubt. Reason perceives that these are primary facts or first principles, neither admitting nor requiring any proof, testified by consciousness, and self-evidently verified on that testimony. By reason, of course is meant the reason of all mankind--that is, of all who are presumably competent to judge on the subject. So that any just or reasonable confidence in the verdict of my own reason--in this or in any other matter, presupposes a due comparison of my own reason with that of others, nay, in some cases, a consideration of the supposably more enlightened reason of future times. I discriminate first principles from derived ones thus:--"I see the sun," is a first principle to me; "you see it," is a first principle to you; by comparing these two ideas, each attains the derived principle that the other sees what he does, and the further derived principle that the sun is an existence independent of both. His own existence is, indeed, to every one the first principle, by means of which he infers the existence of other things and beings. In coming now to the other kind of first principles, consisting of propositions formed by reason, we perceive that these show symptoms of still further difference from the above, than that which results from the difference of their source, of difference that affects their philosophical character, and their technical right to the name under which they present themselves to us. In short, the primary philosophy has not yet settled their title. They are perceived by us to be true by an act of reason called intuition. Not similarly, however, does our reason inform us that they really are first principles, and our science is hitherto unequal to this inquiry. Take, for instance, the following celebrated thesis, so often cited as the most fundamental of all the propositions of reason, insomuch as to be tacitly implied in all our reasonings; which yet we are not sure is a first principle, all that can be said in favor of its pretensions being that we can find no one who is able to reduce it to more primary elements:-- It is impossible for a thing at the same time to be and not to be. Any one agreeing, as every one must, that this is true, might still justly put the query, Why is it impossible? thereby calling its assertion in question, demanding its credentials of proof, seeking some ground for its truth other than its own testimony, and hypothesising some other proposition more fundamental than it of which it would be a derivative, and by all and each of these proceedings, rejecting its claim to be a first principle.