Arimnestus was a philosopher who lived in Greece during the late 5th to early 4th century BCE. He is known only from a single mention in Plato’s dialogue Phaedo, where Socrates recalls a saying he once heard. In the text, Arimnestus is identified as the brother of the well-known Pythagorean thinker Philolaus, which places him within the intellectual circles of Pythagoreanism and early Platonism.

No writings by Arimnestus himself have survived, and no specific works are attributed to him in ancient sources. His entire historical presence rests on being cited by Socrates as the source for a philosophical position: that one must not take one’s own life unless a god sends a necessity to do so. This idea becomes an important starting point for the dialogue’s deeper discussion about the soul, death, and the philosopher’s readiness to die.

According to modern scholars, Arimnestus’s significance lies in this brief role. By citing him, Socrates introduces a Pythagorean perspective on suicide at a critical moment in the conversation. Arimnestus serves as a named authority linking the dialogue’s arguments to Pythagorean teaching, even though nothing else about his life or thought is recorded.