The informations that we perceive are not random, they contain regularities. Mathematically, that means that these informations could be described by an expression smaller than the one describing them trivially. For exemple, the sequence "ABCABCABCABCABCABCABCABCABCABC" which contains 30 characters can be described by the expression "ABC repeated 10 times" which contains only 21 characters. Perceived regularities permit us to suppose that raw perception is the unfolding of a smaller germ (as "ABCABCABCABCABCABCABCABCABC" is the unfolding of "ABC repeated 10 times") and that there exists an outside world which gives our perceptions depending on the actions that we give it. This outside world is not directly accessible but only through this information exchange.
From this information exchange, we build a mental representation of this outside world. So, for example, the child examining his memories realizes that each time that he raises an object and drops it, he saw it go down. He deduces that objects spontaneously tends to go down. So he introduces in his mental representation the concept of gravity force. In the same way, the physicist builds this mental construct further in a more precize and quantitative way, noticing that this object follows a trajectory of equation z = 1/2 g t² + z0. So he builds what he calls the laws of physics, but which are in fact only mathematical models describing more and more precize approximations of reality, but always approximatives and limited to a validity domain depending on the experimental conditions from which the theory has been built.
The regularities of our perceptions appear to us with such a strength that we forget that we perceive only perceptions, that the physical universe, space, time, matter... are only mental constructions.