One friend is a stage, for Seneca.

Satis enim magnum alter alteri theatrum sumus.

Indeed, we are a large enough stage for one another.

Satis: enough, quite

Enim: for, indeed

Magnum: great, large, impressive (agrees with theatrum)

Alter … alteri: one … for the other (reciprocal pronouns)

Theatrum: theatre (literal), stage, spectacle, audience (metaphorical)

Sumus: we are