Most thoroughly, however, did the Stoics—and, in particular, Posidonius [480]—devote themselves to investigating those problems, which may be summed up under the name of meteorology. This portion, however, of their enquiries is of little value for illustrating their philosophical tenets, and it may suffice to mention in a note the objects which it included, and the sources whence information may be obtained. [481] The same treatment may be given to the few maxims laid down by the Stoics on the subject of inorganic nature which have come down to us. [482] Nor need we mention here the somewhat copious writings of Posidonius, [483] on the subjects of geography, history, and mathematics. [(3) Plants and animals.] Little attention was devoted by the Stoics to the world of plants and animals. About this fact there can be no doubt, since we neither hear of any treatises by the Stoics on this subject, nor do they appear to have advanced any peculiar views. The most prominent point is, that they divided all things in nature into four classes—those of inorganic beings, plants, animals, and rational beings. In beings belonging to the first class a simple quality (ἕξις) constitutes the bond of union; in those of the second class, a forming power (φύσις); in those of the third class, a soul; and in those of the fourth class, a rational soul. [484] By means of this division, the various branches of a science of nature were mapped out, based on a gradually increasing development of the powers of life. No serious attempt was made by the Stoics to work out this thought. With the single exception of man, we know exceedingly little of their views on organic beings. [485] CHAPTER IX. THE STUDY OF NATURE. MAN. [A. The soul.] [(1) Materialistic nature of the soul.]