Why is it that we remember with effort but forget without effort? That we learn with effort but stay ignorant without effort? That we are active with effort, and lazy without effort?
 
[Quid est enim, quod cum labore meminimus, sine labore obliuiscimur; cum labore discimus, sine labore nescimus; cum labore strenui, sine labore inertes sumus?]

Augustine of Hippo (354-430) Christian church father, philosopher, saint [b. Aurelius Augustinus]

City of God [De Civitate Dei], Book 22, ch. 22 (22.22) (AD 412-416) [tr. Green (Loeb) (1972)]
    (Source)

(Source (Latin)). Alternate translations:

What is our labour to remember things, our labour to learn, and our ignorance without this labour? our agility got by toil, and our dullness if we neglect it?
[tr. Healey (1610)]

For why is it that we remember with difficulty, and without difficulty forget? learn with difficulty, and without difficulty remain ignorant? are diligent with difficulty, and without difficulty are indolent?
[tr. Dods (1871)]

How difficult it is to remember, how easy to forget; how hard to learn and how easy to be ignorant; how difficult to make an effort and how easy to be lazy.
[tr. Walsh/Honan (1954)]

How is it that what we learn with toil we forget with ease? that it is hard to learn, but easy to be in ignorance? That activity goes against the grain, while indolence is second nature?
[tr. Bettenson (1972)]

Why is it that we remember with such difficulty, but forget so easily? Why is it that we learn with such difficulty, yet so easily remain ignorant? Why is it that we are vigorous with such difficulty, yet so easily inert?
[tr. Dyson (1998)]