Infatuation, hate, homesickness, despair, rapture, neurosis, addiction - what do they all have in common? Aside from neurosis, all are emotions. Aside from rapture and possibly infatuation, all are disagreeable and disabling states of mind. What unites them is that all are mental conditions which so dominate the mind that everything else is excluded. They totally fill the conscious mind.
The conscious mind is a limited entity. We can only think of one thing at a time. Our attention to any given matter can therefore be quantified by measuring the amount of time our conscious mind dwells on that matter. If we sleep eight hours a day, then the size of our mental space is sixteen hours a day.
Rather than speaking of mental space in terms of hours or minutes per day, it is easier to talk in terms of "percent of mental space" devoted to a given endeavor. If a person is seriously neurotic, then he spends nearly sixteen hours a day fixated on his neurosis. This leaves him no time to devote to other issues, since 100% of his mental space is already committed.
The value of the concept of mental space is that it is quantifiable, that it applies to all individuals rather than to some particular class (such as the insane), and that it is easily related to other important concepts. One such concept is that of effectiveness.
Consider the effectiveness of a teenage girl. Much of her mental space -let us say 20% - is occupied by concern over her appearance. (Will the boys notice that new pimple on her cheek? Do her clothes match? If only she hadn't let the stylist cut her hair so short!) Such a teenager is at a disadvantage vis-a-vis someone who does not labor under such concerns. This is true even if the teenager is very bright; she is still that much less effective in devoting attention to other matters. Yet the teenager might be in much the same situation as an old man who devotes 20% of his time worrying about his health.
Although we can only think of one thing at a time, it might be argued that we are capable of shifting our attention quite rapidly, so that we can often apparently do several things at the same time. We can carry on a conversation while we drive, or wash dishes and think about something else. Yet the instances where this is possible are those where one of the activities is some manual task which has been well learned. So although it is possible for the teenage girl to wash dishes while worrying about her appearance, it is not possible for her to do homework while worrying about her appearance. Effectiveness in higher order thinking is much more seriously affected by a shrunken mental space than is the carrying out of daily, habitual tasks.
The concept of mental space allows the seriousness of a disease to be quantified. How do you measure pain? One way would be to give a patient a stopwatch and have him push the button each time he is distracted by pain or illness. The total time the button was depressed is a measure of the seriousness of the problem. It might be argued that this approach makes no allowance for the severity of the pain, but that is not entirely true. If a pain is minor, one thinks about it only when it occurs. If a pain is excruciating, one spends a great deal of time worrying that the pain will recur. If a sick man spends 60% of his time in suffering, that leaves only 40% of his time for enjoyment or useful activity.
Most people would agree that unpleasant or fruitless mental activities should be avoided. The garden of mental space needs constant weeding to root out circular or wishful thinking. Again, the measure of success is the amount of time wasted.