Modern theory is about objects lower than man; even stars, being common things, are lower than man.
There is nothing so practical as a good theory.
Theory is when we know everything but nothing works. Praxis is when everything works but we do not know why. We always end up by combining theory with praxis: nothing works and we do not know why.
It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.
In theory, there is no difference between theory and practice. But, in practice, there is.