noted that this distinction does not entirely coincide with the one we have been insisting upon just now.
III
Verifiability means possibility of verification. Professor Lewis justly remarks that to “omit all examination of the wide range of significance which could attach to ‘possible verification’, would be to leave the whole conception rather obscure” (loc. cit. 137). For our purpose it suffices to distinguish between two of the many ways in which the word 'possibility' is used. We shall call them ‘empirical possibility’ and ‘logical possibility’. Professor Lewis describes two meanings of ‘verifiability’ which correspond exactly to this difference; he is fully aware of it, and there is hardly anything left for me to do but carefully to work out the distinction and show its bearing upon our issue.
I propose to call ‘empirically possible’ anything that does not contradict the laws of nature. This is, I think, the largest sense in which we may speak of empirical possibility; we do not restrict the term to happenings which are not only in accordance with the laws of nature but also with the actual state of the universe (where ‘actual’ might refer to the present moment of our own lives, or to the condition of human beings on this planet, and so forth). If we chose the latter definition (which seems to have been in Professor Lewis’s mind when he spoke of “possible experience as conditioned by the actual”, loc, cit. 141) we should not get the sharp boundaries we need for our present purpose. So ‘empirical possibility’ is to mean ‘compatibility with natural laws’.
Now, since we cannot boast of a complete and sure knowledge of nature’s laws, it is evident that we can never assert with certainty the empirical possibility of any fact, and here we may be permitted to speak of degress of possibility. Is it possible for me to lift this book? Surely! — This table? I think so! — This billiard table? I don’t think so! — This auto-mobile? Certainly not! — It is clear that in these cases the answer is given by experience, as the result of experiments performed in the past. Any
