The saying that things may work nicely in theory, but do not necessarily work in practice is well known. It is typically meant to disparage the importance of theory, suggesting it would be too far removed from practical matters to help in solving the issue at hand.
.. Kant made the point that theory provides "principles of a fairly general nature," or general rules. However, theory does not tell man how to apply it, says Kant. For this, the act of judgment is required:No one can pretend to be practically versed in a branch of knowledge and yet treat theory with scorn, without exposing the fact that he is an ignoramus in his subject.
In this methodological work Ludwig von Mises (1881–1973) emphasized the importance of theory for acting man at the most fundamental level, noting that theory and human action are in fact inseparable. Mises writes,Action is preceded by thinking. Thinking is to deliberate beforehand over future action and to reflect afterwards upon past action. Thinking and acting are inseparable. Every action is always based on a definite idea about causal relations. He who thinks a causal relation thinks a theorem. Action without thinking, practice without theory are unimaginable. The reasoning may be faulty and the theory incorrect; but thinking and theorizing are not lacking in any action. On the other hand thinking is always thinking of a potential action. Even he who thinks of a pure theory assumes that the theory is correct, i.e., that action complying with its content would result in an effect to be expected from its teachings. It is of no relevance for logic whether such action is feasible or not.