Some of the fundamental principles are intuitive and sensible. For example, Newton's law of motion: in the absence of any external influence (i.e. - in the vacuum of deep space), an object in motion will move in a straight line.

Some are counter-intuitive, but easy to grasp. For example, any observer moving along the path of a ray of light will observe that it moves at c=300 million meters per second, the speed of light. This is true even if the observers are moving towards each other at nearly the speed of light. Each will observe light moving at c, but measure different wavelengths.

Some are subtle. In quantum mechanics, uncertainty is introduced in two ways. There is uncertainty in initial state, such as when we observe the behavior of light emitted from a candle. Measurements of such systems manifest a distribution of values that reflect the ensemble statistics of the experimental preparation. Then there is quantum uncertainty, such as Heisenberg's uncertainty principle, which says we cannot precisely measure both position and momentum. Both types of uncertainty are resolved by making many independent measurements. However, the formalism of quantum mechanics does not clearly distinguish the two types of uncertainty, causing some difficulty when attempting to relate interpretations of the formalism applied to experiments on systems manifesting different types of uncertainty.

Some are completely obscure. Quantum mechanics tells us that a particle exhibits "wave-particle" duality. Even physicists are baffled by the mathematical implications of this assertion. Richard Feynman was quoted famously as saying, in effect, that quantum mechanics was a set of mathematical procedures that did not have a self-consistent philosophical interpretation. Attempts to resolve these difficulties have led to nonsensical assertions that consciousness is an active element of quantum phenomena (i.e. - Schroedinger's cat is simultaneously alive and dead until we open the lid of the box).