The entire preoccupation of the physicist is with things that contain within themselves a principle of movement and rest. And to seek for this is to seek for the second kind of principle, that from which comes the beginning of the change. (Aristotle, 340BC)
Physics constitutes a logical system of thought which is in a state of evolution, whose basis (principles) cannot be distilled, as it were, from experience by an inductive method, but can only be arrived at by free invention. The justification (truth content) of the system rests in the verification of the derived propositions (a priori / logical truths) by sense experiences (a posteriori / empirical truths). ... Evolution is proceeding in the direction of increasing simplicity of the logical basis (principles). .. We must always be ready to change these notions - that is to say, the axiomatic basis of physics - in order to do justice to perceived facts in the most perfect way logically. (Albert Einstein, Physics and Reality, 1936)
The supreme task of the physicist is to arrive at those universal elementary laws from which the cosmos can be built up by pure deduction. There is no logical path to these laws; only intuition, resting on sympathetic understanding of experience, can reach them. (Albert Einstein, 1918)
It is the grand object of all theory to make these irreducible elements (axioms/assumptions) as simple and as few in number as possible, without having to renounce the adequate representation of any empirical content whatever. (Albert Einstein, 1954)
The development during the present century is characterized by two theoretical systems essentially independent of each other: the theory of relativity and the quantum theory. The two systems do not directly contradict each other; but they seem little adapted to fusion into one unified theory. For the time being we have to admit that we do not possess any general theoretical basis for physics which can be regarded as its logical foundation. (Albert Einstein, 1940)
If, then, it is true that the axiomatic basis of theoretical physics cannot be extracted from experience but must be freely invented, can we ever hope to find the right way? I answer without hesitation that there is, in my opinion, a right way, and that we are capable of finding it. I hold it true that pure thought can grasp reality, as the ancients dreamed. (Albert Einstein, 1954)
What we observe as material bodies and forces are nothing but shapes and variations in the structure of space. Particles are just schaumkommen (appearances).The world is given to me only once, not one existing and one perceived. Subject and object are only one. The barrier between them cannot be said to have broken down as a result of recent experience in the physical sciences, for this barrier does not exist. (Erwin Schrodinger, 1967)
Before the time of Newton the subject of Physics was called natural philosophy. Now physics is concerned with all aspects of nature, covering the behavior of objects under the action of forces (basically the study of how matter moves around and interacts with other matter in Space). Of the four known forces, gravitational and electromagnetic are long range (act across the universe), and Strong and Weak nuclear forces are short range (10^-15m) and are used to explain nuclear interactions.