3 i6 POPULAR SCIENCE MONTHLY 



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 our- 

 selves 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 ulti- 

 mate 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 phe- 

 nomena B and C. What we know are sensations B' and C produced re- 

 spectively 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 tbunder 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 

 tho first? 



