Association by similarity is the linking together of impressions of a similar kind, irrespective of time and place. Carpenter expresses it as follows: "The law of similarity expresses the general fact that any present state of consciousness tends to revive previous states which are similar to it.... Rational or philosophical association is when a fact or statement on which the attention is fixed is associated with some fact previously known, to which it has a relation, or with some subject which it is calculated to illustrate." And as Kay says: "The similars may be widely apart in space or in time, but they are brought together and associated through their resemblance to each other. Thus, a circumstance of to-day may recall circumstances of a similar nature that occurred perhaps at very different times, and they will become associated together in the mind, so that afterwards the presence of one will tend to recall the others." Abercrombie says of this phase of association: "The habit of correct association--that is, connecting facts in the mind according to their true relations, and to the manner in which they tend to illustrate each other, is one of the principle means of improving the memory, particularly that kind of memory which is an essential quality of a cultivated mind--namely, that which is founded not upon incidental connections, but on true and important relations." As Beattie says: "The more relations or likenesses that we find or can establish between objects, the more easily will the view of one lead us to recollect the rest." And as Kay says: "In order to fix a thing in the memory, we must associate it with something in the mind already, and the more closely that which we wish to remember resembles that with which it is associated, the better is it fixed in the memory, and the more readily is it recalled. If the two strongly resemble each other, or are not to be distinguished from each other, then the association is of the strongest kind.... The memory is able to retain and replace a vastly greater number of ideas, if they are associated or arranged on some principle of similarity, than if they are presented merely as isolated facts. It is not by the multitude of ideas, but the want of arrangement among them, that the memory is burdened and its powers weakened." As Arnott says: "The ignorant man may be said to have charged his hundred hooks of knowledge (to use a rude simile), with single objects, while the informed man makes each hook support a long chain to which thousands of kindred and useful things are attached."