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Consider a particle,
say an electron, moving through space.
We describe the electron's motion in terms
of its position and momentum.
Classically speaking,
we can measure both quantities to infinite precision.
However in Quantum Mechanics,
we can never know both quantities absolutely precisely.
This is because by taking a measurement
we inadvertently have disturbed the system.

"I believe that the existence of the classical 'path'
can be pregnantly formulated as follows:
The "path" comes into existence only when we observe it."
--Heisenberg, in the uncertainty principle paper

If every concept has a meaning only in terms of the experiments used to measure it, we must agree that things that cannot be measured really have no meaning in physics. So for instance, the path of a particle has no meaning beyond the precision with which it is observed. But a basic assumption of physics since Newton has been that a "real world" exists independently of us, regardless of whether or not we observe it. (This assumption did not go unchallenged, however, by some alchemists and philosophers.)
Heisenberg now argued that such concepts as orbits of electrons do not exist in nature unless and until we observe them.
Does this perhaps suggest that all knowledge is unknowable?
There simply are no certainties.
Only probabilities.
Fascinating and supportable thought to say the least.

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