We make our new ideas by merging parts of older ones — and that means keeping more than one idea in mind at once. Let's oversimplify matters for the moment and imagine that the mind is composed of many divisions, each involved with a different activity, like vision, locomotion, language, and so forth. This pattern repeats on smaller scales, so that even the thought of the simplest ordinary object is made up of smaller thoughts in smaller agencies. Thinking about a small white rubber ball could activate some divisions like these:
We'll need some way to talk about the states of many agencies at once. So, in this book, I'll use the expression mental state or total mental state when talking about the states of all of one's agents. The new phrase partial mental state is for talking about the states of smaller groups of agents. Now in order to be clear, we'll have to simplify our picture of the situation, the way scientists do. We shall assume that each agent in our society, at each moment, is either in a quiet state or an active state. Why can't an agent be partially aroused, instead of only on or off? They could indeed, but there are technical reasons why this would not make any fundamental difference to the issues we are discussing here. In any case, this assumption allows us to be precise:
A total state of mind is a list that specifies which agents are active and which are quiet at a certain moment. A partial state of mind merely specifies that certain agents are active but does not say which other agents are quiet.
Notice that according to this definition, a mind can have exactly one total state at any moment, but it can be in many partial states at the same time — because partial states are incomplete descriptions. The picture above shows a mind-society made up of several separate divisions, so we can think of each division's state as one partial state, and this lets us imagine that the entire system can think several thoughts at once, just as a crowd of separate people can. When your speech division is being occupied with what your friend is saying while your vision division looks for a door to exit through — then your mind is in two partial states at once.
The situation is more interesting when two K-lines activate agents in the same division at the same time: imposing two different partial mental states on the same agency can lead to conflicts. It is easy to think of a small white ball because this activates K-lines that connect to unrelated sets of agents. But when you try to imagine a round square, your agents for round and square are forced to compete to control the same set of shape-describing agents. If the conflict is not settled soon, noncompromise may eliminate both — and leave you with the sense of an undefined shape.