Science can be separated from pseudo-science by the principle of falsifiability , the concept that ideas must be capable of being proven false in order to be scientifically valid. Reductionism is the belief that any complex set of phenomena can be defined or explained in terms of a relatively few simple or primitive ones. Reductionism is very similar to, and has its roots from, Occam's Razor , which states that between competing ideas, the simplest theory that fits the facts of a problem is the one that should be selected. Reductionism was widely accepted due to its power in prediction and formulation. The 20th century reaction against reductionism is relativism .
Modern science is somewhere in between.
Another way of stating this is that for everything that happens there are conditions such that, given them, nothing else could happen, the outcome is determined. Determinism implies that everything is predictable given enough information. Newtonian or classical physics is rigidly determinist, both in the predictions of its equations and its foundations, there is no room for chance, surprise or creativity. Everything is as it has to be, which gave rise to the concept of a clockwork Universe . The belief that the underlying order of the Universe can be expressed in mathematical form lies at the heart of science and is rarely questioned.