Now the fact that such an inquiry has originated in the human mind, and that it can not rest satisfied without some solution, is conclusive evidence that the mind has an instinctive belief, a proleptic anticipation, that such knowledge can be attained. There must unquestionably be some mental initiative which is the _motive_ and _guide_ to all philosophical inquiry. We must have some well-grounded conviction, some _à priori_ belief, some pre-cognition "ad intentionem ejus quod quæritur,"[564] which determines the direction of our thinking. The mind does not go to work aimlessly; it asks a specific question; it demands the "_whence_" and the "_why_" of that which is. Neither does it go to work unfurnished with any guiding principles. That which impels the mind to a determinate act of thinking is the possession of a _knowledge_ which is different from, and independent of, the process of thinking itself. "A rational anticipation is, then, the ground of the _prudens quæstio_--"the forethought query, which, in fact, is the prior half of the knowledge sought."[565] If the mind inquire after "laws," and "causes," and "reasons," and "grounds,"--the first principles of all knowledge and of all existence,--"it must have the _à priori_ ideas of "law," and "cause," and "reason," and "being _in se"_ which, though dimly revealed to the mind previous to the discipline of reflection, are yet unconsciously governing its spontaneous modes of thought. The whole process of induction has, then, some rational ground to proceed upon--some principles deeper than science, and more certain than demonstration, which reason contains within itself, and which induction "draws out" into clearer light. [Footnote 564: Bacon.] [Footnote 565: Coleridge, vol. ii. p. 413.] Now this mental initiative of every process of induction is the intuitive and necessary conviction _that there must be a sufficient reason why every thing exists, and why it is as it is, and not otherwise_;[566] or in other words, if any thing begins to be, some thing else must be supposed[567] as the ground, and reason, and cause, and law of its existence. This "_law of sufficient_ (or _determinant) reason_"[568] is the fundamental principle of all metaphysical inquiry. It is contained, at least in a negative form, in that famous maxim of ancient philosophy, "_De nihilo nihil_"--"Ἀδύνατον γίνεσθαί τι ἐκ μηδενὸς προϋπάρχοντος." "It is impossible for a real entity to be made or generated from nothing pre-existing;" or in other words, "nothing can be made or produced without an efficient cause."[569] This principle is also distinctly announced by Plato: "Whatever is generated, is necessarily generated from a certain αἰτίαν"--_ground, reason_, or _cause_; "for it is wholly impossible that any thing should be generated without a cause."[570] [Footnote 566: "Phædo," § 103.] [Footnote 567: _Suppono_, to place under as a support, to take as a ground.]