But there is yet another minimum condition. If my observation of _β_ preceding _α_ is to justify the generalisation that the _existence_ of A is generally preceded by the _existence_ of B, it is plain, I think, that both the _β_ and the _α,_ which I observed, must have _existed_ or been _real_; and that also the existence of _β_ must _really_ have preceded that of _α_. It is plain that if, when I observed _α_ and _β_, _α_ existed but _β_ did not, this observation could give me no reason to suppose that on another occasion when A existed, _β_ _would_ exist. Or again, if, when I observed _β_ preceding _α_, both _β_ and _α_ existed, but the existence of _β_ did not _really_ precede that of α, but, on the contrary, followed it, this observation could certainly give me no reason to suppose that, in general, the existence of A was _preceded_ by the existence of B. Indeed this condition that what is observed must have been _real_ might be said to be included in the very meaning of the word "observation." We should, in this connection, say that we had _not_ observed _β_ preceding _α,_ unless _β_ and _α_ were both real, and _β_ had really preceded _α._ If I say "I have _observed_ that, on one occasion, my hearing of the word 'moon' was followed by my imagining a luminous silvery disc," I commonly mean to include in my statement the assertion that I did, on that occasion, really hear the word "moon," and really did have a visual image of a luminous disc, and that my perception was really followed by my imagination. If it were proved to me that this had not really happened, I should admit that I had not really observed it. But though this condition that, if observation is to give reason for a generalisation, what is observed must be real, may thus be said to be implied in the very word "observation," it was necessary for me to mention the condition explicitly. It was necessary, because, as I shall presently show, we do and must also use the word "observation" in a sense in which the assertion "I observe A" by no means includes the assertion "A exists"--in a sense in which it _may_ be true that though I did observe A, yet A did _not_ exist.