We approach this group of problems from another side when we test the relations of various kinds of machines to various mental types.  Psychologists have studied, for example, the various styles of typewriting machines.[30] From a purely commercial point of view the merits of one or another machine are praised as if they were advantageous for every possible human being.  The fact is that such advantages for one may be disadvantages for another on account of differences in the mental disposition.  One man may write more quickly on one, another on another machine.  As every one knows, the chief difference is that of the keyboard and that of the visible or invisible writing.  Machines like the Remington machine work with a shift key; that is, a special key must be pressed when capital letters are to be written.  Other machines like the Oliver even demand double shifting, one key for the capital letters, and one for the figures, and so on.  On the other hand, machines like the Smith Premier have no shift key, but a double keyboard.  It is evident that both the shift-key arrangement and the double keyboard have their particular psychological advantages.

The single alphabet demands much less from the optical memory, and the corresponding motor inner attitude of consciousness is adjusted to a smaller number of possibilities.  But the pressure on the shift key, which goes with the single alphabet, is not only a time-wasting act; from the psychological point of view it is first of all a very strong interruption of the uniform chain of impulses.  If the capital and small letters are written for a minute alternatingly with the greatest possible speed, the experiment shows that the number of letters for the machine with the double alphabet is about three times greater than for the machine with simple alphabet and shift key.  Both systems accordingly have their psychological advantages and disadvantages.  Human beings of distinct visual ideational type or of highly developed motor type will prefer the double alphabet, provided, of course, that the touch system of writing is learned, and this will be especially true if their inner attitude is easily disturbed by interruptions.  But those who have a feebly developed optical mental centre and who have small ability for the development of complex motor habits will be more efficient on the machines with the single alphabet, especially if their nervous system is little molested by interruptions and thus undisturbed by the intrusion of the shift key act.

In a similar way the visibility of the writing will be for certain individuals the most valuable condition for quick writing, while for others, who depend less upon visual support, it may mean rather a distraction and an interference with the speediest work.  The visible writing attracts the involuntary attention, and thus forces consciousness to stick to that which has been written instead of being concentrated on that which is to be produced by the next writing movements.