Although these two types of attention are discussed as if they were entirely separated from each other, as if one occurred in this situation and the other in that, still as a matter of fact the actual conditions involve an interplay between the two. It is seldom true that free attention is given for any great length of time without flashes of forced attention being scattered through it. Often the forced attention may be needed for certain parts of the work, although as a whole it may take free attention. The same thing is true of occasions when forced attention is used. There are periods in the activity when free attention will carry the worker on. Every activity, then, is likely to be complex so far as the kind of attention used, but it is also characterized by the predominance of one or the other type. The question as to the conditions which call out each type of attention is an important one. As has already been said, free attention is given when the situation attended to satisfies a need. Physiologically stated, free attention is given when a neurone series which is ready to act is called into activity. The situations which do this, other things being equal, will be those which appeal to some instinctive tendency or capacity, or to the self-activity or the personal experience of the individual and which therefore are in accord with his stage of development and his experience. Forced attention is necessary when the neurone tracts used by the attention are for some reason unready to act. Situations to which attention is given through fear of punishment, or when the activity involves a choice of ideal ends as opposed to personal desires, or when some instinctive tendency must be inhibited or its free activity is blocked or interfered with, or when the laws of growth and experience are violated, take forced attention. Of course fatigue, disease, and monotony are frequent breeders of forced attention.