The “avidity to know the causes of things,” that is, to explain them, “is the parent of all philosophy true and false,” says Reid (EIP II, vi [260b]). In particular, this “avidity” is the parent of that false philosophy which is the Way of Ideas. “An object placed at a proper distance, and in a good light, while the eyes are shut, is not perceived at all; but no sooner do we open our eyes upon it, than we have, as it were by inspiration, a certain knowledge of its existence, of its colour, figure, and distance” (ibid.). “This is a fact which every one knows.” “The vulgar are satisfied with knowing the fact,” and don't bother trying to explain it. The philosopher wants “to know how this event is produced, to account for it, or assign its cause” (ibid.).

It was this “avidity” to explain that gave rise to the Way of Ideas. Not all by itself, of course. As with anyone who attempts to explain something, the Way of Ideas theorists operated with convictions as to the principles that an explanation of perception and memory, to be satisfactory, would have to satisfy. It was their attempt to explain, coupled with their commitment to these principles, that drove the Way of Ideas theorists to postulate “ideas” as an essential ingredient in perception and memory.

As we have seen, the account offered for (what is ordinarily called) perception ran, in its essentials, as follows: the conditions for a satisfactory explanation carry the implication that perception cannot incorporate immediate apprehension of some external object; the external object is always too far away for that.