_Possibility of reducing the primary Laws to one comprehensive Principle._--I have already suggested that possibly the primary laws admit of being reduced to some one general and comprehensive principle. This is a point deserving attention. Were we required to name some one principle which should comprehend these several specific laws of association, it would be that of the prior existence in the mind of the suggesting and the suggested idea. The two conceptions have, for some reason, and at some time, stood together before the mind, and hence the one recalls the other. _It seems to be a general law of thought_, _that whatever has been perceived or conceived in connection with some other object of perception or thought, is afterward suggestive of that other._ The relation may be that of part to whole, of resemblance, of contiguity, or contrast, or cause; it may be a natural or an artificial relation; whatever it is that serves as the connecting link between one thought and another, as they come before the mind at first, that will also serve as the ground of subsequent connection, when either of these thoughts shall present itself again to the mind. The one will suggest the other. _Application of this Principle to the several Laws of Suggestion._--Why is it, for example, that things contiguous in time and place suggest each other? In consequence of that contiguity they were viewed by the mind in connection with each other; as, _e. g._, the handle, and the door to which it belongs, the book, and its neighbor on the shelf. It is because Napoleon and his marshals, Wellington and Waterloo, have been presented together to the thoughts, that one now recalls the other. For the same reason the light hair and blue eyes of the person passing in the street recall the friend of former years; that peculiarity of hair and of eyes has been, in my mind, previously connected with the conception of my friend. So also a part suggests the whole with which it has been ordinarily connected, as, for example, the crystal and the watch. _Further Application of the same Principle._--On the same principle cause and effect are naturally suggestive. We have been accustomed to observe the elision of a spark in connection with the forcible collision of flint and steel and whenever we have observed the application of fire to gunpowder, certain consequences have uniformly attracted our attention; hence the one of these things awakens immediately in our minds the conception of the other, with which it has previously coëxisted. For the same reason the instrument suggests the idea of the wound, and the wound of the instrument. The sight of a rose, and the sensation of fragrance, have usually coëxisted; hence either recalls the other.