From Poincare, 1903:

A very small cause that
escapes our notice determines a considerable effect that we cannot fall to 
see, and then we say that the effect is
due to chance. If we knew exactly the laws of nature and the situation of 
the universe at the initial moment, we could
predict exactly the situation of that same universe at a succeeding moment. 
But even if it were the case that the
natural laws had no longer any secret for us, we could still only know the 
initial situation approximately. If that
enabled us to predict the succeeding situation with the same approximation, 
that is all we require, and we should
say that the phenomenon had been predicted, that it is governed by laws. 
But it is not always so; it may happen that
small differences in the initial conditions produce very great ones in the 
final phenomena. A small error in the former
will produce an enormous error in the latter. Prediction becomes 
impossible, and we have the fortuitous
phenomenon.

Ref: Ivars Peterson, Newton's Clock, p. 167