Biology Reference
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whereas in a probabilistic system several events are possible, each
having a probability between 0 and 1.
This apparent simplicity conceals enormous difficulties. I said
for example, “If I let go of this pen I am holding in my hand, I have
absolutely no doubt that it will fall and that that is what will hap-
pen every time I let it go in the future.” Such an affirmation poses
the problem of induction raised by David Hume (1711-1776). What
logic allows us to be certain that an event that is produced in cer-
tain conditions will necessarily be reproduced in the future? The
sun rose today as it does every day but that does not prove that it
will rise tomorrow. Yet science is constructed based on this type of
inductive reasoning whenever we formulate universal laws from par-
ticular experiences. Indeed, we seem to acquire such a high degree
of belief in the fact that the experience will repeat itself that it
attains a degree of absolute certainty. Probability would therefore
seem to be a measure of the degree of belief we have in certain
events occurring, in relation to our level of knowledge or ignorance
of these events. This is the subjective conception of probability first
proposed by Hume (1739) and developed subsequently by numer-
ous scientific philosophers in different forms.
However, there is another conception of probability, known as
objective or frequency probability. We are aware that certain phe-
nomena are produced with a certain constant frequency when the
same event is repeated a very great number of times, for example,
the coin falls tails up in 50% of cases. It really seems that it demon-
strates an intrinsic property which arises from the physical struc-
ture of the coin. According to the objective conception, probability
would seem to reflect this intrinsic property of the coin. It does not
here reflect our ignorance but is an objective property of the world.
Several philosophers have tried to present theories for this objective
existence of chance. For many authors, the two aspects of proba-
bility, subjective and objective, have always coexisted (Hacking, 1975;
Martin, in press). Debates on this question are fraught with diffi-
culties and are not the subject of this topic. We shall simply state
that the objective existence of chance today seems to have been
affirmed by quantum physics and that Karl Popper (1902-1994)
