_Criteria of primary Truths._--It becomes an important inquiry, in what manner we may recognize and distinguish first truths from all others. Besides common consent, or universality of belief on the part of those who have arrived at years of discretion, _Buffier_ relies, also, upon the following, as criteria of first principles; that they are such truths as can neither be defended nor attacked by any propositions, either more manifest or more certain than themselves; and that their practical influence extends even to those who would deny them. Reid gives, among other criteria, the following: consent of ages and nations; the absurdity of the opposite; early appearance in the mind, prior to education and reasoning; practical necessity to the conduct and concerns of life. _Hamilton_ gives the following as tests or criteria of first truths: 1. _Incomprehensibilty._--We comprehend that the thing is, but not _how_ or _why_ it is. 2. _Simplicity._--If the cognition or belief can be resolved into several cognitions or beliefs, it is complex, and so, no longer original. 3. _Necessity, and consequent universality._--If necessary, it is universal, and if absolutely universal, then it must be necessary. 4. _Comparative evidence and certainty._ _Summary of Criteria._--The following may be regarded as a summary of the more important criteria by which to distinguish primary truths from all others. _a._ As first truths, or primary data of intelligence, they are, of course, not derived from observation or experience, but are prior and necessary to such experience. _b._ They are _simple_ truths, not resolvable into some prior and comprehending truth from which they may be deduced. _c._ As simple truths, they _do not admit of proof_, there being nothing more certain which can be brought in evidence of them. _d._ While they do not admit of proof, _the denial of them involves us in absurdity_. _e._ Accordingly, as simple, and as self-evident, they are universally admitted. _Enumeration of some of the Truths usually regarded as primary._--Different writers have included some more, some fewer, of these first principles in their list; while no one has professed, so far as I am aware, to give a complete enumeration of them. Such an enumeration, if it were possible, would be of great service in philosophy. The following have been generally included among primary truths by those who have attempted any specification, viz.; our personal existence, our personal identity, the existence of efficient causes, the existence of the material world, the uniformity of nature; to which would be added, by others, the reliability of memory, and of our natural faculties generally, and personal freedom or power over our own actions and volitions.