The natural sciences, especially physical science, have made very great strides in this century. I refer especially to Einstein’s theory of relativity and the quantum theory. There can be no doubt that these theories are valid; that is to say, either they are true, or they are abstractions bearing such a relation to truth that they systematise observations and lead to correct predictions. Among scientists theories are no longer regarded as true but have come to be recognised as abstractions, as a result of the teachings of Mach, but even more so as a result of the impact of relativity and quantum theory. The naïve realism of Newtonian physics is dead. But at the same time nobody has succeeded in building up a system of natural philosophy showing exactly how theoretical science is related to reality. The effect of this has been, on the one hand, to move scientists towards scepticism or even a kind of Kantian idealism, as in the case of the late Sir Arthur Eddington, doubting or denying the validity of the concepts of reality and truth. On the other hand it has prevented the Thomist arguments for the existence of God from resting upon the conclusions of natural science through the mediation of natural philosophy. Instead they rest upon the nature of common-sense.