But the greatest undertakings should not be overly pondered, lest contemplation of difficulties too clearly foreseen appall you.

[Los grandes empeños aun no se han de pensar, basta ofrecerse, porque la dificultad, advertida, no ocasione el reparo.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 204 (1647) [tr. Fischer (1937)]
    (Source)

(Source (Spanish)). Alternate translations:

As to great enterprizes, we must not stand reasoning, it is enough that we embrace them when they present, lest the consideration of their difficulty make us abandon the attempt.
[Flesher ed. (1685)]

Great undertakings are not to be brooded over, lest their difficulty when seen causes despair.
[tr. Jacobs (1892)]

In moments of great danger, don't even think, simply act. Don't dwell on the difficulties.
[tr. Maurer (1992)]