He shows that ideas suggest each other, not according to relation among their objects, but among the ideas themselves.He dwells on what he calls the law of precedence." One idea acquires power to suggest another by immediately preceding it," using the word "power" in the sense in which it is commonly used in physical inquiries.It follows, (1) "If one idea acquire power to suggest another by immediately preceding it, the greater the number of ideas that it immediately precedes, the greater the number it will acquire the power of suggesting." (2) If one idea acquire power to suggest others by immediately preceding them, the more frequently it precedes, the greater power it must acquire to suggest them.He thus explains the circumstance that, when we meet with a person whom we have formerly seen only in one particular place, there is usually, to our recollection, a very distinct idea of that place; but when we meet with a person whom we have seen in a great many different places, there is seldom recalled an idea of any of them.(3) If an idea acquire power to suggest others by immediately preceding them, the greater the number of ideas that immediately precede any others, the greater will be their power when they recur to suggest these ideas.(4) If an idea acquire power to suggest another by immediately preceding it, the more vivid the idea that precedes any other, the greater will be its power when it recurs in a state equally vivid to suggest it.
He says that the law of contiguity in point of time is really three laws, -- {392} that "an idea will acquire power to suggest another by immediately preceding it, by existing at the same with it, and by immediately following it.The first of these laws is that of precedence, and into it he resolves all the others.Thus he resolves the law of coexistence into precedence." Let A and B be two ideas which coexist for two sensible points of time, then A, while existing in the first point, precedes B while B is existing in the second; and B, while existing in the first point, precedes A while A is existing in the second point."Proceeding on the principle that the greater the number of ideas that coexist with any other, the greater the number afterwards suggested by i@ he explains how the longer an idea continues in the mind, the more readily will it afterwards recur; how the more frequently it has been in the mind, it will come up the more readily; how the longer it continues, it will be the more likely to recur; and how the more frequently it has been in the mind, it will be the more likely to continue.He resolves in the same way the law of contiguity in place, and the law of cause and effect; and affirms that the idea of cause has no power, independently of the law of precedence, to suggest the idea of an effect.
He tries hard to explain in the same way the law of similarity." Yesterday I saw a winged animal, to-day a winged animal of the same species." "When I yesterday saw the first animal, I obtained ideas of its peculiar qualities, and likewise of those common to it with all the individuals of the species to which it belonged.To-day, when I saw the other animal, I also obtained ideas of its peculiar properties, and of those common to it with all the species to which it belonged, that is, along with the ideas of the peculiar properties of the second animal, I obtained a number of ideas which coexisted with those of the peculiar properties of the first.According to the doctrine of coexistence, formerly explained, they would suggest ideas of the peculiar properties of the first.When they do so, Ihave ideas both of the common and of the peculiar properties of the first animal; in other words, I have an idea of the first animal itself, for that idea can be nothing but the aggregate of the ideas of its common and peculiar properties." He explains the law of contrast by showing that, in every case where contrasted ideas suggest one another, it will be found that there is a considerable degree of similarity along with the contrast.He accounts by the same law of precedence for the secondary laws of Brown.
Some of these resolutions seem to me over subtle, but they are worthy of consideration by those who would sound the depths of the subject.
He treats at length of the voluntary principle, and offers many judicious remarks.He criticises Stewart's doctrine of power, according to which " the author of nature has bestowed on matter no powers at all, of course never preserves its powers in being, nor even employs them in accomplishing his purposes, as there are no powers to be exercised.In the second volume (Part I 1.), he treats of moral law, of right, jurisprudence, and politics, somewhat after the manner of Stewart, with considerable sweep of style, but no great power of metaphysical analysis.{393}