Of science Aristotle takes for his examples sometimes celestial physics, more often geometry or arithmetic, sometimes a concrete science, e.g. botany.[61] In the field of pure form, free from the disconcerting surprises of sensible matter and so of absolute necessity, no difficulty arises as to the deducibility of the whole body of a science from its first principles. In the sphere of abstract form, mathematics, the like may be allowed, abstraction being treated as an elimination of matter from the [Greek: synolon] by one act. When we take into account relative matter, however, and traces of a conception of abstraction as admitting of degree,[62] the question is not free from difficulty. In the sphere of the concrete sciences where law obtains only [Greek: hôs epi to polu] this ideal of science can clearly find only a relative satisfaction with large reserves. In any case, however, the problem as to first principles remains fundamental. Formal and scientific principles. Induction and dialectic. Knowledge and reality. Conclusions as to induction. If we reject the infinite regress and the circle in proof (_circulus in probando_) which resolves itself ultimately into proving A by B and B by A,[63] we are confronted by the need for principles of two kinds, those which condition all search for truth, and those which are the peculiar or proper principles of special sciences, their "positions," viz. the definitions of their subjects and the postulates of the existence of these. All are indemonstrable and cannot be less sure than the body of doctrine that flows from them. They must indeed be recognized as true, primary, causative and the like. But[64] they are not congenitally present in the individual in a determinate shape. The doctrine of latency is mystical and savours of Plato's reminiscence (_anamnesis_). Yet they must have something to develop from, and thereupon Aristotle gives an account of a process in the psychological mechanism which he illustrates by comparative psychology, wherein a [Greek: logos] or meaning emerges, a "first" universal recognized by induction. Yet [Greek: nous], intelligence, is the principle of first principles. It is infallible, while, whatever the case with perception of the special sensibles,[65] the process which combines particulars is not. On the side of induction we find that experience is said to give the specific principles,[66] "the phenomena being apprehended in sufficiency." On the side of intuition, self-evidence of scientific principles is spoken of.[67] Yet dialectic is auxiliary and of methodological importance in their establishment.[68] Mutually limiting statements occur almost or quite side by side. We cannot take first principles "as the bare precipitate of a progressively refined analysis"[69] nor on the other as constitutive a priori forms. The solution seems to lie in the conception of a process that has a double aspect. On the one hand we have