In this talk, we introduce the idea by G.N. Hounsfield  for the practicalization of the computerized tomography (CT). Godfrey Newbold Hounsfield (1919-2004) was an English electrical engineer. He developed an idea to practicalize CT, for which he shared the 1979 Nobel Prize for Physiology or Medicine with Allan McLeod Cormack. It being a pity that G.N. Hounsfield left no paper on his idea to practicalize CT, in this talk, we shall introduce what is believed to be his idea. He considered a simple discretization of the mathematical model for CT and simplified the problem of CT to obtain a problem to solve a system of linear equations having no solution. The essence of the idea by G.N. Hounsfield is  how to obtain such a system of linear equations having no solution and how to solve it, which shall be introduced in this talk. Although his idea worked well in practice, almost all procedures for the reconstruction are not theoretically rationalized. We shall also discuss how to prove these procedures to be theoretically right.