Full Idea
The knowledge of the position of a particle is complementary to the knowledge of its velocity or momentum. If we know one with high accuracy we cannot know the other with high accuracy; still we must know both for determining the behaviour of the system.
Gist of Idea
Position is complementary to velocity or momentum, so the whole system is indeterminate
Source
Werner Heisenberg (Physics and Philosophy [1958], 03)
Book Reference
Heisenberg,Werner: 'Physics and Philosophy' [Penguin 1989], p.37
A Reaction
This is the famous Uncertainty Principle, expressed in plain language by the man himself. At this point we lost our grip on the prospects of determining the behaviour of natural systems.