1. The ultimate material antecedents of thought are found in impressions which are due to external objects as stimuli. Taken in themselves, these impressions are mere psychical states or events. They exist in us side by side, or one after the other, according as the objects which excite them operate simultaneously or successively. The occurrence of these various psychical states is not, however, entirely dependent upon the presence of the exciting thing. After a state has once been excited, it gets the power of reawakening other states which have accompanied it or followed it. The associative mechanism of revival plays a part. If we had a complete knowledge of both the stimulating object and its effects, and of the details of the associative mechanism, we should be able from given data to predict the whole course of any given train or current of ideas (for the impressions as conjoined simultaneously or successively become ideas and a current of ideas). Taken in itself, a sensation or impression is nothing but a "state of our consciousness, a mood of ourselves." Any given current of ideas is a necessary sequence of existences (just as necessary as any succession of material events), happening in some particular sensitive soul or organism. "Just because, under their respective conditions, every such series of ideas hangs together by the same necessity and law as every other, there would be no ground for making any such distinction of value as that between truth and untruth, thus placing one group in opposition to all the others."[13] 2. Thus far, as the last quotation clearly indicates, there is no question of reflective thought, and hence no question of logical theory. But further examination reveals a peculiar property of the current of ideas. Some ideas are merely coincident, while others may be termed coherent. That is to say, the exciting causes of some of our simultaneous and successive ideas really belong together; while in other cases they simply happen to act at the same time, without there being a real connection between them. By the associative mechanism, however, both the coherent and the merely coincident combinations recur. The first type of recurrence supplies positive material for knowledge; the second gives occasion for error.