Variant: A friend is one soul abiding in two bodies.
Variant: Love is composed of a single soul inhabiting two bodies.
Source: The Lives and Opinions of Eminent Philosophers, p. 188; also reported in various sources as:
Friendship is a single soul dwelling in two bodies.
A true friend is one soul in two bodies.
Love is composed of a single soul inhabiting two bodies.
What is a friend? A single soul dwelling in two bodies.

“Those who know, do. Those that understand, teach.”

Aristotle

This and many similar quotes with the same general meaning are misattributed to Aristotle as a result of Twitter attribution decay. The original source of the quote remains anonymous. The oldest reference resides in the works of George Bernard Shaw, Man and Superman (1903): "Maxims for Revolutionists", where he claims that “He who can, does. He who cannot, teaches.”. However, the related quote, "Those who can, do. Those who understand, teach" likely originates from Lee Shulman in his explanation of Aristotlean views on professional mastery: Source: Shulman, L. S. (1986). Those who understand: Knowledge growth in teaching. Educational Researcher, 15(2), 4 - 14. Stable URL: http://www.jstor.org/stable/1175860
Misattributed
Variant: Those who can, do, those who cannot, teach.

“Courage is the first of human qualities because it is the quality which guarantees the others.”

Aristotle

“What is the essence of life? To serve others and to do good.”

Aristotle

Often given as a saying of Aristotle with no reference.
Disputed

“The roots of education … are bitter, but the fruit is sweet.”

Aristotle

The Lives and Opinions of Eminent Philosophers

“The energy of the mind is the essence of life.”

Aristotle

Source: The Philosophy of Aristotle

“He who has overcome his fears will truly be free.”

Aristotle

“Knowing yourself is the beginning of all wisdom.”

Aristotle

“Nature does nothing uselessly.”

Aristotle book Politics

Book I, 1253a.8
Source: Politics

“Choice, not chance, determines your destiny.”

Aristotle

“It is during our darkest moments that we must focus to see the light.”

Aristotle

“In all things of nature there is something of the marvelous.”

Aristotle book Parts of Animals

Book I, 645a.16
Parts of Animals

“If, then, God is always in that good state in which we sometimes are, this compels our wonder; and if in a better this compels it yet more. And God is in a better state. And life also belongs to God; for the actuality of thought is life, and God is that actuality; and God's self-dependent actuality is life most good and eternal.”

Aristotle book Metaphysics

Book XII, 1072b.24
Metaphysics
Original: (el) εἰ οὖν οὕτως εὖ ἔχει, ὡς ἡμεῖς ποτέ, ὁ θεὸς ἀεί, θαυμαστόν: εἰ δὲ μᾶλλον, ἔτι θαυμασιώτερον. ἔχει δὲ ὧδε. καὶ ζωὴ δέ γε ὑπάρχει: ἡ γὰρ νοῦ ἐνέργεια ζωή, ἐκεῖνος δὲ ἡ ἐνέργεια: ἐνέργεια δὲ ἡ καθ᾽ αὑτὴν ἐκείνου ζωὴ ἀρίστη καὶ ἀΐδιος.
Source: https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0051%3Abook%3D12%3Asection%3D1072b

“For the purposes of poetry a convincing impossibility is preferable to an unconvincing possibility.”

Aristotle book Poetics

1461b.11
Poetics