I hear thunder, and I conclude there has been an electric discharge; I do not hesitate to consider the physical phenomenon as anterior to the auditory image perceived in my consciousness, because I believe it is its cause. Behold then the rule we follow, and the only one we can follow: when a phenomenon appears to us as the cause of another, we regard it as anterior. It is therefore by cause that we define time; but most often, when two facts appear to us bound by a constant relation, how do we recognize which is the cause and which the effect? We assume that the anterior fact, the antecedent, is the cause of the other, of the consequent. It is then by time that we define cause. How save ourselves from this _petitio principii_? We say now _post hoc, ergo propter hoc_; now _propter hoc, ergo post hoc_; shall we escape from this vicious circle? X Let us see, not how we succeed in escaping, for we do not completely succeed, but how we try to escape. I execute a voluntary act _A_ and I feel afterward a sensation _D_, which I regard as a consequence of the act _A_; on the other hand, for whatever reason, I infer that this consequence is not immediate, but that outside my consciousness two facts _B_ and _C_, which I have not witnessed, have happened, and in such a way that _B_ is the effect of _A_, that _C_ is the effect of _B_, and _D_ of _C_. But why? If I think I have reason to regard the four facts _A_, _B_, _C_, _D_, as bound to one another by a causal connection, why range them in the causal order _A B C D_, and at the same time in the chronologic order _A B C D_, rather than in any other order? I clearly see that in the act _A_ I have the feeling of having been active, while in undergoing the sensation _D_ I have that of having been passive. This is why I regard _A_ as the initial cause and _D_ as the ultimate effect; this is why I put _A_ at the beginning of the chain and _D_ at the end; but why put _B_ before _C_ rather than _C_ before _B_? If this question is put, the reply ordinarily is: we know that it is _B_ which is the cause of _C_ because we always see _B_ happen before _C_. These two phenomena, when witnessed, happen in a certain order; when analogous phenomena happen without witness, there is no reason to invert this order. Doubtless, but take care; we never know directly the physical phenomena _B_ and _C_. What we know are sensations _B'_ and _C'_ produced respectively by _B_ and _C_. Our consciousness tells us immediately that _B'_ precedes _C'_ and we suppose that _B_ and _C_ succeed one another in the same order. This rule appears in fact very natural, and yet we are often led to depart from it. We hear the sound of the thunder only some seconds after the electric discharge of the cloud. Of two flashes of lightning, the one distant, the other near, can not the first be anterior to the second, even though the sound of the second comes to us before that of the first? XI