of fictions, forcibly adjusted to reality, but is to trace back phenomena to their real grounds), obedience to which enabled him to deduce _a priori_ from causes the conclusions which Copernicus by fortunate conjecture had gathered inductively from effects--these made our thinker a forerunner of Newton. The physical method of explanation must not be corrupted either by theological conceptions (comets are entirely natural phenomena!) or by anthropomorphic views, which endow nature with spiritual powers. [Footnote 1: See Sigwart, _Kleine Schriften_, vol. i. p. 182 _seq_.; R. Eucken, _Beiträge zur Geschichte der neueren Philosophie_, p. 54 _seq_.] [Footnote 2: Aristotle erred when he considered qualitative distinctions (_idem_ and _aliud_) ultimate. These are to be traced back to quantitative differences, and the _aliud_ or _diversum_ is to be replaced by _plus et minus_. There is nothing absolutely light, but only relatively. Since all things are distinguished only by "more or less," the possibility of mediating members or proportions between them is given.] Intermediate between Bacon and Descartes, both in the order of time and in the order of fact, and a co-founder of modern philosophy, stands Galileo Galilei (1564-1641).[1] Galileo exhibits all the traits characteristic of modern thinking: the reference from words to things, from memory to perception and thought, from authority to self-ascertained principles, from chance opinion, arbitrary opinion, and the traditional doctrines of the schools, to "knowledge," that is, to one's own, well grounded, indisputable insight, from the study of human affairs to the study of nature. Study Aristotle, but do not become his slave; instead of yielding yourselves captive to his views, use your own eyes; do not believe that the mind remains unproductive unless it allies itself with the understanding of another; copy nature, not copies merely! He equals Bacon in his high estimation of sensuous experience in contrast to the often illusory conclusions of the reason, and of the value of induction; but he does not conceal from himself the fact that observation is merely the first step in the process of cognition, leaving the chief rôle for the understanding. This, supplementing the defect of experience--the impossibility of observing all cases--by its _a priori_ concept of law and with its inferences overstepping the bounds of experience, first makes induction possible, brings the facts established into connection (their combination under laws is thought, not experience), reduces them to their primary, simple, unchangeable, and necessary causes by abstraction from contingent circumstances, regulates perception, corrects sense-illusions, _i. e_., the false judgments originating in experience, and decides concerning the reality or fallaciousness of phenomena. Demonstration based on experience, a close union of observation and thought, of fact and Idea (law)--these