Jean Piaget, The Psychology of Intelligence, Translated from the French by Malcolm Piercy and D. E. Berlyne, The International Library of Psychology, Philosophy and Scientific Method, Routledge and Kegan Paul, 1950
Chapter V, “The Growth of Thought - Intuitions and Operations”, pp. 120-121:
As regards its scope, sensori-motor intelligence deals only with real entities, and each of its actions thus involves only very short distances between subject and objects. It is doubtless capable of detours and reversals, but it never concerns anything but responses actually carried out and real objects. Thought alone breaks away from these short distances and physical pathways, so that it may seek to embrace the whole universe including what is invisible and sometimes even what cannot be pictured; this infinite expansion of spatio- temporal distances between subject and objects comprises the principal innovation of conceptual intelligence and the specific power that enables it to bring about operations.
There are thus three essential conditions for the transition from the sensori-motor level to the reflective level. Firstly, an increase in speed allowing the knowledge of the successive phases of an action to be moulded into one simultaneous whole. Next, an awareness, not simply of the desired results of action, but its actual mechanisms, thus enabling the search for the solution to be combined with a consciousness of its nature. Finally, an increase in distances, enabling actions affecting real entities to be extended by symbolic actions affecting symbolic representations and thus going beyond the limits of near space and time.