Bringing together now what we have learned regarding the higher and more difficult forms of attention, as revealed by sustained attention and work under distraction, by the span of attention and by trying to do two things at once, we find the previously stated three laws of attention further illustrated, and a couple of new laws making their appearance. (1) The _law of selection_ still holds good in these more {263} difficult performances, since only one attentive response is made at the same instant of time. Automatic activities may be simultaneously going on, but any two attentive responses seem to be inconsistent with each other, so that the making of one excludes the other, in accordance with the general law of selection. What shall we say, however, of reading four disconnected letters at the same time, or of seeing clearly four colors at the same time? Here, it would seem, several things are separately attended to at once. The several things are similar, and close together, and the responses required are all simple and much alike. Such responses, under such very favorable conditions, are perhaps, then, not inconsistent with each other, so that two, three, or even four such attentive responses may be made at the same time. (2) The _law of advantage_ holds good, as illustrated by the fact that some distractions are harder to resist than others. (3) The _law of shifting_ holds good, as illustrated by the constant movement of attention, even when it is "sustained", and by the alternation between two activities when we are trying to carry them both along simultaneously. (4) The _law of sustained attention_, or of _tendency_ in attention, is the same old law of tendency that has shown itself repeatedly in earlier chapters. A tendency, when aroused to activity, facilitates responses that are in its line and inhibits others. A tendency is thus a strong factor of advantage, and it limits the shifting of attention. (5) A new law has come to light, the _law of combination_, which reads as follows: _a single response may be made to two or more stimuli_; or, _two or more stimuli may arouse a single joint response_. Even though, in accordance with the law of selection, only one attentive response is made at the same time, more than {264} one stimulus may be dealt with by this single attentive response. Groups of four dots are grasped as units, familiar words are grasped as units. Notice that these units are our own units, not external units. Physically, a row of six dots is as much a unit as a row of four, but we grasp the four as a unit in a way that we cannot apply to the six. Physically, six letters are as much a unit when they do not form a word as when they do; but we can make a unitary response to the six in the one case and not in the other. The response is a unit, though aroused by a number of separate stimuli.