4 
a case of cause and effect, and each of the five 
has its characteristic. 
Night follows day. But we do not say that 
day is the cause of night. As night follows 
day, so day follows night, and the sequence 
is reversible, and we have day, night, 
day, night, and so on in periodic suc- 
cession. But in such a succession we 
cannot say that day is the antecedent of 
night any more than that night is the antece- 
dent of day, for we might with equal propriety 
write the series thus : — Night, day, night, day, 
and so on. In such cases we have a cyclical se- 
quence. Thus summer and winter, flow and 
ebb of tide, are cyclical sequences ; and we do 
not say that summer is the cause of winter, or 
flow the cause of ebb. One characteristic of 
many of these cyclical sequences seems to be 
that we could have no conception of one mem- 
ber of the cycle unless we had a conception of 
the other. Thus we could have no conception 
of winter, night, or ebbing tide, unless we had 
a conception of summer, day, and flowing tide ; 
nor any conception of summer, day, and flow, 
unless we had one of winter, night and ebb. 
We might look out on a frozen and 
darkened landscape, but we should not 
know it was winter and night unless we knew 
there was a summer and day. Rising water 
would not be to us a flowing tide, unless we 
knew that there was such a thing as an ebbing 
tide. Liquid lead would not be called melted 
lead unless lead, unlike quicksilver, was solid at 
ordinary temperatures. Cyclical sequences 
point, not so much to a mutual physical depen- 
dency of the events, as to their common physical 
origin. 
Death follows life. But we do not say that 
life is the cause of death. This sequence is not 
cyclical. We cannot reverse it, and say that 
life follows death, for life follows birth. The 
sequence may be termed formal. A thing is 
not said to be the cause of its own end, nor is 
the beginning of a thing said to be cause either 
of the thing itself or of its end. Birth and 
life, life and death, living birth and death are 
all formal sequences I think the rule is, that 
whenever we cannot conceive a material part of 
either, no matter which, member of a sequence 
without a conception of a material part of the 
other, and the sequence is not reversible, then 
we may call it formal. My reason is, that 
physical antecedents and subsequents are only 
known by experience, and are in general utterly 
dissimilar and without any logical connection. 
If one member of a sequence is the reverse of 
the other, or ii both only contain one physi- 
cal conception, this kind of formal sequence 
may be termed spurious. The light of a 
meteor momentarily illumines a dark 
sky. We may call the change from 
darkness to light the antecedent, and that 
from light to darkness the subsequent. There 
are two events, but only one physical object — 
viz., the light. Inquiry would aim at determining 
the origin of the light, and not, properly speak- 
ing, the connection between the light and the 
darkness. If the spurious sequence were cycli- 
cal, as in the case of any periodic light, research 
would take the same direction. For a physical 
cause, we look outside a formal sequence. 
Thunder follows lightning. In one view this 
seems to be an ordinary sequence j in another, 
a sequence in the nature of a concurrence. If 
we say that the thunder is audible after the 
lightning has been visible, we leave open the 
question, whether we ought not to recur to two 
simultaneous initial disturbances, having a com- 
mon source, and from one of which the light, 
and from the other the sound, was propagated. 
We may compare the lightning and thunder 
with the electric spark and snap, which re- 
semble it, and with the flash and report 
of fire arms, which are analogous to it. If 
we come to the conclusion that the 
initial disturbances were simultaneous, and 
that, in their inception, the light and sound 
issued simultaneously from a common source, in 
the same way that light and heat issue from 
flame, we ought not to say that the light and 
sound are cause and effect. Simultaneous or 
concurrent events indeed cannot stand in that 
relation, for they lack the beforeness and after- 
ness involved in every definition of cause and 
effect. Two simultaneous events, though they 
can have no such mutual dependence, may, or 
may not, have a common physical origin on 
which each depends. The shadow on a sun- 
dial, and the hands of a chronometer, simul- 
taneously mark the hours. But the motions of 
the shadow, and those of the hands, have no 
mutual dependence, and no common physical 
origin. Again, seeming concurrences may be 
real sequences. Thus, the rotation of a steam 
paddle is the cause, that of a waterwheel the 
effect, of a current of water. 
Violent atmospheric disturbance follows a 
great and Budden fall of mercury in the 
barometer. But we do not say that the 
fall of the mercury is the cause of 
the disturbance. This sequence could 
be destroyed by causing a similar 
fall by artificial means, in which case the atmo- 
spheric disturbance would not follow. We need 
not actually try the experiment, for well in- 
formed persons would not expect the sequence. 
Sequences like this may be called destructible, 
and are in the nature, not of cause and effect, 
but of mere precursor and principal event. If 
aa instrument were so contrived as to be affected 
by a north wind only, and to give a conspicuous 
indication so long as a north wind blew on it, 
and no longer, the indication might be 
differently regarded by different persons. To 
an observer near the instrument the indication 
would be a sign that the wind was in the north. 
To one considerably south of the instrument the 
indication would be preeursive. Should the 
