The third book of the Tetrabiblos contains a chapter on the quality of the soul, and it opens with a division that shaped every later astrological account of the person. The soul has two parts. The rational part is governed by Mercury. The sensitive or irrational part is governed by the Moon.
The division is not Ptolemy's invention. It is the standard philosophical anatomy of his period, and he is applying astrology to a psychology already in general circulation. What astrology adds is a way of examining each part separately. To read the rational soul, examine Mercury: its sign, its aspects, whether it is direct or retrograde, and which planets configure to it. To read the sensitive soul, examine the Moon in the same way. A chart in which the two are in good condition and in aspect describes a person whose thinking and whose feeling are in communication. A chart in which they are unrelated describes one in which they are not.
The scheme carries a further refinement. Ptolemy takes the planets that aspect Mercury and the Moon as the ones that colour each part, so that Saturn makes the rational soul careful and hard, Jupiter makes it expansive, Mars quick, Venus soft. The two significators supply the faculties and the aspecting planets supply the character.
What the chapter does not do is offer a single significator for the soul as a whole. That is a later development, and this entry keeps the two accounts apart.