Of the universal distribution of the elementary constituents of mind Clifford writes as follows: "That element of which, as we have seen, even the simplest feeling is a complex, I shall call _Mind-stuff_. A moving molecule of inorganic matter does not possess mind or consciousness; but it possesses a small piece of mind-stuff. When molecules are so combined together as to form the film on the under side of a jellyfish, the elements of mind-stuff which go along with them are so combined as to form the faint beginnings of Sentience. When the molecules are so combined as to form the brain and nervous system of a vertebrate, the corresponding elements of mind-stuff are so combined as to form some kind of consciousness; that is to say, changes in the complex which take place at the same time get so linked together that the repetition of one implies the repetition of the other. When matter takes the complex form of a living human brain, the corresponding mind-stuff takes the form of a human consciousness, having intelligence and volition." This is the famous mind-stuff doctrine. It is not a scientific doctrine, for it rests on wholly unproved assumptions. It is a play of the speculative fancy, and has its source in the author's strong desire to fit mental phenomena into some general evolutionary scheme. As he is a parallelist, and cannot make of physical phenomena and of mental one single series of causes and effects, he must attain his end by making the mental series complete and independent in itself. To do this, he is forced to make several very startling assumptions:-- (1) We have seen that there is evidence that there is consciousness somewhere--it is revealed by certain bodies. Clifford assumes consciousness, or rather its raw material, _mind-stuff_, to be everywhere. For this assumption we have not a whit of evidence. (2) To make of the stuff thus attained a satisfactory evolutionary series, he is compelled to assume that mental phenomena are related to each other much as physical phenomena are related to each other. This notion he had from Spinoza, who held that, just as all that takes place in the physical world must be accounted for by a reference to physical causes, so all happenings in the world of ideas must be accounted for by a reference to mental causes, _i.e._ to ideas. For this assumption there is no more evidence than for the former. (3) Finally, to bring the mental phenomena we are familiar with, sensations of color, sound, touch, taste, etc., into this evolutionary scheme, he is forced to assume that all such mental phenomena are made up of elements which do not belong to these classes at all, of something that "cannot even be felt." For this assumption there is as little evidence as there is for the other two.