(2) The semi-sciences, incompletely proved (certain portions of biology, psychology, sociology, etc.), although they show a regression of imaginative explanation repulsed by the hitherto absent or insufficient experimentation, nevertheless abound in hypotheses, that succeed, contradict, destroy one another. It is a commonplace truism that does not need to be dwelt on--they furnish _ad libitum_ examples of what has been rightly termed scientific mythology. Aside from the quantity of imagination expended, often without great profit, there is another character to be noted--the nature of the belief that accompanies imaginative creation. We have already seen repeatedly that the intensity of the imaginary conception is in direct ratio to the accompanying belief, or rather, that the two phenomena are really one--merely the two aspects of one and the same state of consciousness. But faith--i.e., the adherence of the mind to an undemonstrated assertion--is here at its maximum. There are in the sciences hypotheses that are not believed in, that are preserved for their didactic usefulness, because they furnish a simple and convenient method of explanation. Thus the "properties of matter" (heat, electricity, magnetism, etc.), regarded by physicists as distinct qualities even in the first half of the last century; the "two electric fluids;" cohesion, affinity, etc., in chemistry--these are some of the convenient and admitted expressions to which, however, we attach no explanatory value. There is also to be mentioned the hypothesis held as an approximation of reality--this is the truly scientific position. It is accompanied by a provisional and ever-revocable belief. This is admitted, in principle at least, by all scientists, and has been put into practice by many of them. Lastly, there is the hypothesis regarded as the truth itself--one that is accompanied by a complete, absolute, belief. But daily observation and history show us that in the realm of embryonic and ill-proven sciences this disposition is more flourishing than anywhere else. _The less proof there is, the more we believe._ This attitude, however wrong from the standpoint of the logician, seems to the psychologist natural. The mind clings tenaciously to the hypothesis because the latter is its own creation, or, because in adopting it, it seems to the mind that it should have itself discovered the hypothesis, so much does the latter harmonize with its inner states. Let us take the hypothesis of evolution, for example: we need not mention its high philosophical bearing, and the immense influence that it exerts on almost all forms of human thought. Nevertheless, it still remains an hypothesis; but for many it is an indisputable and inviolable dogma, raised far above all controversy. They accept it with the uncompromising fervor of believers: a new proof of the underlying connection between imagination and belief--they increase and decrease _pari passu_. III