Plato, Roman herm probably copied from a Greek original, 4th century BC. In the Staatliche Museen, Berlin.

Plato was born, the son of Ariston and Perictione, in Athens, or perhaps in Aegina, in about 428 BC, the year after the death of the great statesman Pericles. His family, on both sides, was among the most distinguished in Athens. Ariston is said to have claimed descent from the god Poseidon through Codrus, the last king of Athens; on the mother's side, the family was related to the early Greek lawmaker Solon. Nothing is known about Plato's father's death. It is assumed that he died when Plato was a boy. Perictione apparently married as her second husband her uncle Pyrilampes, a prominent supporter of Pericles; and Plato was probably brought up chiefly in his house. Critias and Charmides, leaders among the extremists of the oligarchic terror of 404, were, respectively, cousin and brother of Perictione; both were friends of Socrates, and through them Plato must have known the philosopher from boyhood.

The most important formative influence to which the young Plato was exposed was Socrates. It does not appear, however, that Plato belonged as a "disciple" to the circle of Socrates' intimates. The Seventh Letter speaks of Socrates not as a "master" but as an older "friend," for whose character Plato had a profound respect; and he has recorded his own absence (through indisposition) from the death scene of the Phaedo. It may well be that his own vocation to philosophy dawned on him only afterward, as he reflected on the treatment of Socrates by the democratic leaders. Plato owed to Socrates his commitment to philosophy, his rational method, and his concern for ethical questions. Among other philosophical influences the most significant were those of Heracleitus and his followers, who disparaged the phenomenal world as an arena of constant change and flux, and of the Pythagoreans, with whose metaphysical and mystical notions Plato had great sympathy.

Plato's Theory of Forms:

Plato believed that there exists an immaterial Universe of `forms', perfect aspects of everyday things such as a table, bird, and ideas/emotions, joy, action, etc. The objects and ideas in our material world are `shadows' of the forms (see Plato's Allegory of the Cave).

This solves the problem of how objects in the material world are all distinct (no two tables are exactly the same) yet they all have `tableness' in common. There are different objects reflecting the `tableness' from the Universe of Forms. Plato refused to write his own metaphysics, knowledge of its final shape has to be derived from hints in the dialogues and statements by Aristotle and, to a far lesser extent, other ancient authorities. According to these, Plato's doctrine of Forms was, in its general character, highly mathematical, the Forms being somehow identified with, or explained in terms of, numbers. Here may be seen the influence of the Pythagoreans, though, as Aristotle says, the details of Plato's views on the mathematical constituents of being were not the same as theirs. In addition Aristotle states that Plato introduced a class of "mathematicals," or "intermediates," positioned between sensible objects and Forms. These differ from sensible objects in being immaterial (e.g., the geometer's triangles ABC and XYZ) and from the Forms in being plural, unlike the Triangle itself.

Aristotle himself had little use for this sort of mathematical metaphysics and rejected Plato's doctrine of transcendent eternal Forms altogether. Something of Platonism, nonetheless, survived in Aristotle's system in his beliefs that the reality of anything lay in a changeless (though wholly immanent) form or essence comprehensible and definable by reason and that the highest realities were eternal, immaterial, changeless self-sufficient intellects which caused the ordered movement of the universe. It was the desire to give expression to their transcendent perfection that kept the heavenly spheres rotating. Man's intellect at its highest was akin to them. This Aristotelian doctrine of Intellect (nous) was easily recombined with Platonism in later antiquity.

Plato's cosmology derives from a mathematical discover by Empedocles. He found that there are only five solid shapes whose sides are made from regular polygons (triangles, squares, pentagons, hexagons, etc) - for example, the cube.

Plato was so impressed with this discovery that he was convinced that atoms of matter must derive from these five fundamental solids. But at the time the Greek periodic table consisted only of earth, water, air and fire (i.e. four atomic types). Therefore, Plato postulated that a fifth atomic type must exist which Aristotle later called `ether'. The heavens, and objects in the heavens (stars, planets, Sun) are composed of atoms of ether.

This is perhaps the first example of the use of theoretical thought experiments to predict or postulate new concepts. In this case, the existence of a new form of matter, ether. And led to the formulation of a Universe that looked like the following:

Excerpt from the Encyclopedia Britannica without permission.