The Ionian school includes the Earlier Ionians, -- Thales,
Anaximander, and Anaximenes, -- and the Later Ionians, whose
proper historical place is after the Eleatic school.
Life. Thales, the first philosopher of Greece, was of Phoenician
descent. He was born at Miletus, about the year 620 B.C. He was a
contemporary of Croesus and Solon, and was counted among the Seven Wise
Men. He is said to have died in the year 546 B.C.
 On the manner of computing the date of Thales, cf. Burnet,
op. cit., pp. 36 ff.
Sources. Our knowledge of the doctrines of Thales is based
entirely on secondary sources, especially on the account given by
Aristotle in Met., I, 3, 983. cf. Ritter and Preller, op. cit.,
According to Aristotle, Thales taught that out of water all things are
made.  Historical tradition is silent as to the reasons by which
Thales was led to this conclusion. It is possible, as Aristotle
conjectures, that the founder of the Ionian school was influenced by
the consideration of the moisture of nutriment, etc.; he may have based
his conclusion on a rationalistic interpretation of the myth of
Oceanus, or he may have observed the alluvial deposits of the rivers of
his native country, and concluded that, as earth, so all things else
come from water. The saying that
"The magnet has a soul because it attracts iron" is attributed to
Thales on the authority of Aristotle, who, however, speaks
conditionally, "if, indeed, he said," etc. We must not attach
importance to Cicero's Stoical interpretation of Thales: "Thales
Milesius aquam dixit esse initium rerum, Deum, autem, eam mentem quae
ex aqua cuncta fingeret." Such a dualism belongs to the time of
Anaxagoras. Similarly, the saying that "All things are full of gods"
(panta plŕra the˘n) is but the expression, in
Aristotle's own phraseology, of the general doctrine of animism, or
hylozoism, which is a tenet common to all the Earlier Ionians. They
maintained that matter is instinct with life; or, as an Aristotelian
would say, they did not distinguish between the material principle and
the formal principle of life.
 Met., I, 3, 983 b.
Life. Anaximander, who was also a native of Miletus, was born
about the year 610 B.C. Theophrastus describes him as a disciple, or
associate, of Thales. The date of his death is unknown.
Sources. Primary sources. Anaximander composed a treatise, or
rather a poetical prose composition, peri phusi˘s, which
was extant when Theophrastus wrote. Of this work two sentences only
have come down to us:
1. "All things must in equity again decline into that whence they have
their origin, for they must give satisfaction and atonement for
injustice, each in order of time." 
 Theophr., frag. 2, apud Diels, Doxographi, p. 476.
2. The infinite "surrounds all things and directs all things." 
 Arist., Phys., III, 4, 203 b.
Secondary sources. Our chief secondary sources are Theophrastus
(in the work phusik˘n doxai, of which the existing
fragments are published by Diels, op. cit., p.476) and Aristotle
(especially in Met., XII, 2, 1069 b; Phys.. III, 4 203
From our secondary sources it is evident that, according to
Anaximander, the originating principle (archŕ)  of all
things is the Infinite, or rather the Unlimited
(apeiron). The reasons, however, which led to this conclusion
are merely a matter of conjecture, as in the case of Thales'
generalization. According to Aristotle, Anaximander, supposing that
change destroys matter, argued that, unless the substratum of change is
limitless, change must sometime cease. Thus, while modern physics holds
that matter is indestructible, Anaximander maintained that it is
infinite; for there can be no question as to the corporeal nature of
the apeiron it is an infinite material substance. Critics, however,
do not agree as to how Anaximander would have answered the questions,
Is the unlimited an element or a mixture of elements? Is it
qualitatively simple or complex? He certainly maintained that the
primitive substance is infinite, but did not, so far as we know,
concern himself with the question of its qualitative determinations.
 "That Anaximander called this something by the name of
phusis is clear from the doxographers; the current statement
that the word arche, in the sense of a 'first principle,' was
introduced by him, is probably due to a mere misunderstanding of what
Theophrastos says." Burnet, op. cit., p. 52.
On the meaning of phusis in the writings of the early Greek
philosophers, cf.Philosophical Review (July, 1901), Vol.
X, pp. 366 ff.
The apeiron has been likened to the modern notion of space and
to the mythological concept of chaos. It is described by Anaximander
himself as surrounding and directing all things, and by Aristotle it is
described as to theion. We must not, however, attach to these
expressions a dualistic or pantheistic meaning.
From the Boundless all things came, by a process which the
Placita  describes as separation (apokrithŕnai). Living
things sprang from the original moisture of the earth (through the
agency of heat). The first animals were therefore fishes, which after
they came on shore threw off their scales and assumed new shapes. Man,
too, was generated from other kinds of animals.  Anaximander is
generally believed to have taught an infinity of worlds.
 Cf. Burnet, op. cit., pp. 372 ff.
 Plut., Strom., 2, apud Diels, op. cit., p. 579.
Historical Position. Comparing the doctrines of Anaximander with
what we know of the teachings of Thales, we find that the former are
far richer in their contents and betoken a higher development of
speculative thought. They represent a higher grade of abstraction, as
is evident in the substitution of the Boundless for the concrete
Life. Anaximenes of Miletus, who was an "associate" of
Anaximander, composed a treatise the title of which is unknown. He died
about 528 B.C.
Sources. Primary sources. The only fragment of the work
of Anaximenes which has survived is a sentence quoted in the
Placita. "Just as our soul, being air, holds us together, so do
breath and air encompass the world." 
 Placita, I, 3, 4, apud Diels, op. cit., p.
278. The Placita, or Placita Philosophorum, is a
collection of the "opinions" of philosophers ascribed to Plutarch. Like
the Eclogae of Stobaeus, it is based on an earlier collection of
opinions called AEtii Placita, as this is in turn based on the
Vetusta Placita, of which traces are found in Cicero.
(cf. Burnet, op. cit., p. 372.)
Secondary sources. Our principal secondary source is
Theophrastus, whom pseudo-Plutarch, Eusebius (Praepratio
Evangelica ), Hippolytus (Refutatio Omnium Heresium), etc.,
follow. cf. Diels, op. cit., p. 476.
According to all our secondary sources, Anaximenes taught that the
principle, or ground, of all material existence is air.
(aŕr must, however, be taken in the Homeric sense of
vapor, or mist.) This substance, to which is ascribed infinite
quantity, is endowed with life. From it, by thinning
(arai˘sis) and thickening (puke˘sis), were
formed fire, winds, clouds, water, and earth.
The world is an animal, whose breathing is kept up by masses of air,
which it inhales from the infinite space beyond the heavens.
Cicero incorrectly represents Anaximenes as identifying the divinity
with the primitive Air. St. Augustine is more correct when he says,
"Nec deos negavit aut tacuit, non tamen ab ipsis aerem factum, sed ipsos
ex aere ortos credidit." 
 De Civ. Dei, VIII, 2.
Historical Position. Anaximenes was evidently influenced by his
predecessors. From Thales he derived the qualitative determinateness of
the primitive substance and from Anaximander its infinity. The doctrine
of "thickening" and "thinning" is far more intelligible than the
doctrine of "separating" which Anaximander taught.
Retrospect. The Early Ionian philosophers were students of
nature (physiologoi) who devoted themselves to the inquiry into
the origin of things. They agreed (1) in positing the existence of a
single original substance; (2) in regarding this substance as endowed
with force and life (hylozoism). They were dynamists. Heraclitus, a
Later Ionian, who was in final analysis a dynamist also, marks the
transition from the early hylozoism to the mechanism of the Later