The distinction between persons and animals on the one side and mere things on the other seems to rest in the last resort on an important practical consideration. Among the things which, according to the naïve Realism of the pre-scientific theory, form my environment, there are some which regularly behave in much the same general way in response to very different types of behaviour on my own part. There are others again which behave differently towards me according to the differences in my behaviour towards them. In other words, some things exhibit special individual purposes, dependent in various ways on the nature of my own individual purposes, others do not. Hence for practice it becomes very important to know what things can be counted on always to exhibit the same general type of behaviour, and what cannot, but require individual study before I can tell how they will respond to different purposive behaviour of my own. It is on this practical difference that the distinction of mental and conscious from purely physical and unconscious existence seems to be based. We shall probably not be far wrong in interpreting the _unconsciousness_ of purely material existence to mean that it exhibits no traces of purposive individuality, or at least none that we can recognise as such. More briefly, the physical order consists of the things which do not manifest recognisable individuality. (_b_) Closely connected with this peculiarity is a second. The physical order is made up of events which conform rigidly to certain universal _Laws_. This is an obvious consequence of its lack of purposive individuality. The elements of which it is composed, being devoid of all purposive character of their own, always behave in the same surroundings in the same regular uniform way. Hence we can formulate precise general Laws of their behaviour. Originally, no doubt, this uniformity of the physical order is thought of as a point of contrast with the irregular behaviour of purposive beings, who respond differently to the same external surroundings according as their own internal purposes vary. With the growth of Psychology as an experimental science of mental processes there inevitably arises the tendency to extend this concept of uniform conformity with general Law to the processes of the psychical order, and we are then confronted by the famous problem how to reconcile scientific law with human “freedom.” The same antithesis between the apparently regular and purposeless behaviour of the elements of the physical order and the apparently irregular and purposive behaviour of the members of the psychical order is also expressed by saying that the sequence of events in the physical order is _mechanically_ determined by the principle of Causality, whereas that of the psychical order is _teleological_, _i.e._ determined by reference to _end_ or purpose.