The aids to memory supposed to be furnished by the various mnemonic systems may now be briefly considered. These methods of supplying the defects of a naturally weak memory, or of strengthening a fairly good one, are one and all artificial. This might not be a conclusive objection to them, were they really effective and permanent helps, enabling one who has learned them to recall with certainty ideas, names, dates, and events which he is unable to recall by other means. Theory apart, it is conceded that a system of memorizing which had proved widely or generally successful in making a good memory out of a poor one, would deserve much credit. But experience with these systems has as yet failed to show, by the stern test of practical utility, that they can give substantial (and still less permanent) aid in curing the defects of memory. Most of the systems of mnemonics that have been invented are constructed on the principle of locality, or of utilizing objects which appeal to the sight. There is nothing new in these methods, for the principle is as old as Simonides, who lived in the fifth century before Christ, and who devised a system of memorizing by locality. One of the most prevalent systems now taught is to select a number of rooms in a house (in the mind's eye, of course) and divide the walls and the floors of each room into nine equal parts or squares, three in a row. Then "On the front wall--that opposite the entrance of the first room--are the units, on the right-hand wall the tens, on the left hand the twenties, on the fourth wall the thirties, and on the floor the forties. Numbers 10, 20, 30, and 40, each find a place on the roof above their respective walls. One room will thus furnish 50 places, and ten rooms as many as 500, while 50 occupies the centre of the roof. Having fixed these clearly in the mind, so as to be able readily and at once to tell the exact position of each place or number, it is then necessary to associate with each of them some familiar object (or symbol) so that the object being suggested, its place may be instantly remembered, or when the place is before the mind, its object may immediately spring up. When this has been done thoroughly, the objects can be run over in any order from beginning to end, or from end to beginning, or the place of any particular one can at once be given. All that is further necessary is to associate the ideas we wish to remember with the objects in the various places, by which means they are readily remembered, and can be gone over in any order. In this way, one may learn to repeat several hundred disconnected words or ideas in any order, after hearing them only once."