Heraclitus believed that the universe is in a constant state of flux and change. He viewed fire as the fundamental essence and driving force behind all change and transformation in the world. In contrast, Parmenides challenged Heraclitus' idea that change is possible. Through rational arguments, Parmenides concluded that reality is singular, eternal, and unchanging being. For Parmenides, all existence and truth must conform to rational thought, and the senses provide an illusion of change and plurality not supported by reason.

1. Problem of Nature
Eleatic School: Parmenides – Heraclitus

2. Heraclitus
• Heraclitus. The fundamental thought in the teaching of Heraclitus is,
as we have already seen, that the universe is in a state of ceaseless
change.
• “One cannot step twice into the same river,” for other and yet other
waters are ever flowing on.
• Fire and the universal flux. To signalize the notion of incessant activity,
Heraclitus chooses as his first principle the most mobile substance he
knows, something that never seems to come to rest, the ever-living
fire—sometimes called by hint vapor or breath—which is regarded by
him as the vital principle in the organism and the essence of the soul.

3. Heraclitus
• The fire of Heraclitus is not the abiding substratum of his
predecessors; it is that which is constantly being transformed into
other things.
• Fire changes into water and then into earth, and the earth changes
back again into water and fire, “ for the way upward and the way
downward are one.”
• “All things are exchanged for fire, and fire for all things; as wares are
exchanged for gold and gold for wares.”
• Things seem to be permanent because we do not perceive the
incessant movements in them, and because what they lose in one
way they gain in another.

4. Heraclitus
• Even the sun is new every day, kindled at its rising and quenched at its
setting.
• Opposites and their union. The primal unity itself is in constant
motion and change; its creation is destruction, its destruction
creation.
• When it passes into something else, e.g., from fire into 'water, the fire
is lost in a new form of existence.
• Everything is changed into its opposite and everything, therefore, is a
union of opposite qualities.

5. Heraclitus
• There are no persistent qualities and hence nothing remains
permanent by virtue of its qualities.
• Everything both is and is not; the universal process is a transition
from one condition to its opposite and, in this sense, everything
unites opposites within itself.
• Such opposition alone makes the world possible.
• Harmony in music, for example, results from the combination of high
notes and low notes, i.e., from a union of opposites.

6. Parmenides
• Parmenides’ ontology. Parmenides was the metaphysician of the Eleatic
school.
• He challenged Heraclitus’ teaching that everything changes, that fire
becomes water, and water earth, and earth fire, that things first are and
then are not.
• How is this possible, he asks, how can a thing both be and not be?
• How can anyone think such a contradiction, how can a thing change its
qualities, how can one quality become another quality?
• To say that it can, is to say that something is and something is not, that
something can come from nothing, and that something can become
nothing.

7. Parmenides
• Or, to employ another line of argument: if being has become, it must
either have come from non-being or from being.
• If from non-being, it has come from nothing, which is impossible; if
from being, then it has come from itself, which is equivalent to saying
that it is identical with itself, and thus has always been.
• It is evident, then, that from being only being can come, that nothing
can become something else, that whatever is always has been and
always will be, that everything remains what it is.
• Hence, there can be only one eternal, underived, unchangeable
being.

8. Parmenides
• Since it is all alike and there cannot be anything in it but being, it must
be continuous and indivisible.
• There can be no break in it, for if the break is real, it is itself being and
so being is continuous after all; whereas if the break is not real, it is
non-being, and again being is continuous.
• Further, it must be immovable, for there is no non-being (empty
space) for it to move in.
• Moreover, being and thought are one, for what cannot be thought,
cannot be; and what cannot be, i.e., non-being, cannot be thought.

9. Parmenides
• That is, thought and being are identical; whatever is thought has
being.
• Thus Parmenides may be characterized as a rational or dialectical
idealist.
• Parmenides may also have believed that being and thought are one in
the sense that reality is endowed with mind; but there is scarcely
sufficient evidence to warrant ascribing to him this mentalistic type of
idealism.

10. Parmenides
• Being or reality is a homogeneous, continuous, indeterminate mass—
which the esthetic imagination of our philosopher pictures as a
sphere— endowed with reason, eternal and immutable.
• All change is inconceivable, and, therefore, the world of sense is an
illusion.
• To regard as true what we perceive by the senses is to confuse being
with non-being.
• Parmenides shows a firm belief in reason: reality is conformable to
reason and what is contradictory to thought cannot be real.