The idea of Infinity, to take one of his most celebrated resolutions, is merely a simple mode of Quantity, as Immensity is a simple mode of Space, and Eternity of Duration. All alike are negative ideas, arising whenever we allow the mind "an endless progression of thought," without any effort to arrest it. "How often soever" a man doubles an unit of space, be it a "mile, or diameter of the earth, or of the _Orbis Magnus_," or any otherwise multiplies it, "he finds that, after he has continued this doubling in his thoughts and enlarged his idea as much as he pleases, he has no more reason to stop, nor is one jot nearer the end of such addition, than he was at first setting out; the power of enlarging his idea of Space by farther additions remaining still the same, he hence takes idea of infinite space." (Bk. II., ch. xvii., § 3.) With the idea of "Substance" Locke is fairly baffled. If we examine our idea of a horse, a man, a piece of gold, &c., we are able to resolve it into a number of simple ideas, such as extension, figure, solidity, weight, colour, &., co-existing together. But, according to Locke, who, in this respect, was merely following in the track of the generally received philosophy of his time, there is, in addition to all these qualities, a _substratum_ in which they inhere, or, to use his own language, "wherein they do subsist, and from which they do result." Now of the various qualities we can form a clear idea and give a more or less intelligible account. But can we form a clear idea or give an intelligible account of the substratum? Locke here is bold enough to break off from the orthodox doctrine of the time, and confess candidly that we cannot. The idea of this Substratum or Substance is a "confused idea of something to which the qualities belong, and in which they subsist." The name Substance denotes a Support, "though it be certain we have no clear or distinct idea of that thing we suppose a support."