Why science classified into different subject?

Answer 1

Mainly because each subject focusses on a different aspect of inquiry. However, although subjects such as Physics, Chemistry, Biology, and Psychology all cover different aspects of the world we live in (i.e., what you learn in a Chemistry class will be very different than what you learn in a Psychology class), there is a huge amount of overlap between the different subjects. Atomic physics, for example, overlaps greatly with chemistry, and the biology of the brain includes a study of the psychology of sensation and perception. In fact, advanced University courses will often reflect this overlap, such as courses in Biochemistry, Psychobiology, and Psychophysics.