What I wish to suggest is that this condition is a universal condition for sound inductions. If the observation of β preceding _α_ can ever give us any reason at all for supposing that the existence of A is generally preceded by that of B, it can at most only give us reason to suppose that the existence of an A is generally preceded by that of a B _which stands to our A in the same relation in which β_ has been observed to stand to _α._ It cannot give the least reason for supposing that the existence of an A must have been preceded by that of a B, which did _not_ stand to A in the observed relation, but in some quite different one. If we are to have any reason to infer from the existence of an A the existence of such a B, the reason must lie in some different observations. That this is so, in the case of hens' eggs and hens, is, I think, obvious: and, if the rule is _not_ universal, some reason should at least be given for supposing that it does apply in one case and not in another. Having thus attempted to point out some conditions which seem to be necessary, though not _sufficient_, where observation is to give any reason for a generalisation, I may now proceed to my second preliminary question. What kinds of things do we observe? In order to illustrate how much and how little I mean by "observation" or "direct perception," I will take as an instance a very common visual perception. Most of us are familiar with the experience which we should describe by saying that we had seen a red book and a blue book side by side upon a shelf. What exactly can we be said to observe or directly perceive when we have such an experience? We certainly observe one colour, which we call blue, and a different colour, which we call red; each of these we observe as having a particular size and shape; and we observe also these two coloured patches as having to one another the spatial relation which we express by saying they are side by side. All this we certainly see or directly perceive _now,_ whatever may have been the process by which we have come to perceive so much. But when we say, as in ordinary talk we should, that the objects we perceive are _books,_ we certainly mean to ascribe to them properties, which, in a sense which we all understand, are not actually seen by us, at the moment when we are merely looking at two books on a shelf two yards off. And all such properties I mean to exclude as not being then _observed_ or _directly perceived_ by us. When I speak of what we _observe,_ when we see two books on a shelf, I mean to limit the expression to that which is _actually seen._ And, thus understood, the expression does include colours, and the size and shape of colours, and spatial relations in three dimensions between these patches of colour, but it includes nothing else.