we could have done, what we did not do, it is, I think, very difficult to be quite sure that this (or something similar) is not what we mean by the word “could.” The case of the ship may seem to be an exception, because it is certainly not true that she would have steamed twenty knots if she had chosen; but even here it seems possible that what we mean is simply that she would, if the men on board of her had chosen. There are certainly good reasons for thinking that we very often mean by “could” merely “would, if so and so had chosen.” And if so, then we have a sense of the word “could” in which the fact that we often could have done what we did not do, is perfectly compatible with the principle that everything has a cause: for to say that, if I had performed a certain act of will, I should have done something which I did not do, in no way contradicts this principle.
And an additional reason for supposing that this is what we often mean by “could,” and one which is also a reason why it is important to insist on the obvious fact that we very often really should have acted differently, if
