Who casts to write a living line, must sweat.

The quote insists that truly vital, enduring writing (“a living line”) requires hard work (“must sweat”). Jonson rejects the romantic notion of effortless genius and frames literary excellence as the product of deliberate striving—drafting, revising, and honing language until it has force and life. The verb “casts” suggests purposeful intention: anyone who sets out to create something that will last must accept the labor involved. The line also carries an ethical undertone common in Jonson’s criticism: art is a discipline, and the writer’s responsibility is to earn clarity, precision, and vitality through exertion rather than relying on spontaneity alone.