Seneca emphasises that philosophy is to be lived

Zenonem Cleanthes non expressisset, si tantummodo audisset.

Cleanthes would not have imitated Zeno if he had merely heard him

Zenonem: Zeno (accusative singular)

Cleanthes: proper noun (nominative singular)

Non: not

Expressisset: would have imitated

Si: if

Tantummodo: only, merely

Audisset: had heard