We ought in fairness to fight our case with no help beyond the bare facts: nothing, therefore, should matter except the proof of those facts. Still, as has been already said, other things affect the result considerably, owing to the defects of our hearers.

[δίκαιον γὰρ αὐτοῖς ἀγωνίζεσθαι τοῖς πράγμασιν, ὥστε τἆλλα ἔξω τοῦ ἀποδεῖξαι περίεργα ἐστίν: ἀλλ᾽ ὅμως μέγα δύναται, καθάπερ εἴρηται, διὰ τὴν τοῦ ἀκροατοῦ μοχθηρίαν.]

Aristotle (384-322 BC) Greek philosopher

Rhetoric [Ῥητορική; Ars Rhetorica], Book 3, ch. 1, sec. 5 (3.1.5) / 1404a (350 BC) [tr. Roberts (1924)]
    (Source)

On style vs. substance in shaping judgment. (Source (Greek)). Alternate translations:

For justice would be to contend with the facts only, so that every thing else beside the mere demonstration is superfluous; nevertheless, it [style] is of great influence, as has been said, owing to the corruption of the hearers.
[Source (1847)]

Our facts ought to be our sole weapons, making everything superfluous which is outside the proof; owing to the infirmities of the hearer, however, style, as we have said, can do much.
[tr. Jebb (1873)]

For justice should consist in fighting the case with the facts alone, so that everything else that is beside demonstration is superfluous; nevertheless, as we have just said, it [style] is of great importance owing to the corruption of the hearer.
[tr. Freese (1926)]

Although [...] in justice, litigants should appeal only to the facts to contest the case, so that everything apart from demonstration is superfluous, it remains the case, as I have aid, that, thanks to the audience's moral weakness, delivery is very effective.
[tr. Waterfield (2018)]