That certainty which direct perception gives us respecting coexistences and sequences of the simplest and most accessible kind, science gives us respecting coexistences and sequences, complex in their dependencies, or inaccessible to immediate observation. In brief, regarded from this point of view, science may be called _an extension of the perceptions by means of reasoning_. On further considering the matter, however, it will perhaps be felt that this definition does not express the whole fact—that inseparable as science may be from common knowledge, and completely as we may fill up the gap between the simplest previsions of the child and the most recondite ones of the physicist, by interposing a series of previsions in which the complexity of reasoning involved is greater and greater, there is yet a difference between the two beyond that above described. And this is true. But the difference is still not such as enables us to draw the assumed line of demarcation. It is a difference not between common knowledge and scientific knowledge; but between the successive phases of science itself, or knowledge itself—whichever we choose to call it. In its earlier phases science attains only to _certainty_ of foresight; in its later phases it further attains to _completeness_. We begin by discovering _a_ relation; we end by discovering _the_ relation. Our first achievement is to foretell the _kind_ {5} of phenomenon which will occur under specified conditions; our last achievement is to foretell not only the kind but the _amount_. Or, to reduce the proposition to its most definite form—undeveloped science is _qualitative_ prevision; developed science is _quantitative_ prevision.