Another very familiar illustration may be adduced. Some persons have what is called an antipathy to a spider, a toad, or a rat. These feelings generally originate in some early fright. The idea of danger has been on some occasion so intensely excited along with the touch or sight of the animal, and hence the association so strongly formed, that it cannot be dissolved. The sensation, in spite of them, excites the idea, and produces the uneasiness which the idea imports. The following of one idea after another idea, or after a sensation, so certainly that we cannot prevent the combination, nor avoid having the _consequent_ feeling as often as we have the _antecedent_, is a law of association, the operation of which we shall afterwards find to be extensive, and bearing a principal part in {97} some of the most important phenomena of the human mind. As there are some ideas so intimately blended by association, that it is not in our power to separate them; there seem to be others, which it is not in our power to combine. Dr. Brown, in exposing some errors of his predecessors, with respect to the acquired perceptions of sight, observes: "I cannot blend my notions of the two surfaces, a plane, and a convex, as one surface, both plane and convex, more than I can think of a whole which is less than a fraction of itself, or a square of which the sides are not equal." The case, here, appears to be, that a strong association excludes whatever is opposite to it. I cannot associate the two ideas of assafoetida, and the taste of sugar. Why? Because the idea of assafoetida is so strongly associated with the idea of another taste, that the idea of that other taste rises in combination with the idea of assafoetida, and of course the idea of sugar does not rise. I have one idea associated with the word pain. Why can I not associate pleasure with the word pain? Because another indissoluble association springs up, and excludes it. This is, therefore, only a case of indissoluble association; but one of much importance, as we shall find when we come to the exposition of some of the more complicated of our mental phenomena.[30] [Editor's footnote 30: Some further elucidation seems needful of what is here said, in so summary a manner, respecting ideas which it is not in our power to combine: an inability which it is essential to the analysis of some of the more complex phenomena of mind that we should understand the meaning of. The explanation is indicated, but hardly more than indicated, in the text.