Learning without thought ends in a blur. Thought without learning will soon totter.
 
[學而不思則罔、思而不學則殆。]

Confucius (c. 551- c. 479 BC) Chinese philosopher, sage, politician [孔夫子 (Kǒng Fūzǐ, K'ung Fu-tzu, K'ung Fu Tse), 孔子 (Kǒngzǐ, Chungni), 孔丘 (Kǒng Qiū, K'ung Ch'iu)]

The Analects [論語, 论语, Lúnyǔ], Book 2, verse 15 (2.15) (6th C. BC – AD 3rd C.) [tr. Ware (1950)]
    (Source)

Many (but not all) translators suggest that learning/study here is not general academics, but examining and maintaining the ancient traditions.

(Source (Chinese)). Alternate translations:

Learning without thought is labour lost; thought without learning is perilous.
[tr. Legge (1861)]

Learning with [sic] thought is a snare; thought without learning is a danger.
[tr. Jennings (1895)]

Study without thinking is labour lost. Thinking without study is perilous.
[tr. Ku Hung-Ming (1898)]

Learning without thought is useless. Thought without learning is dangerous.
[tr. Soothill (1910)]

Education without meditation is useless. Meditation without education is risky.
[tr. Soothill (1910), alternate]

Research without thought is a mere net and entanglement: thought without gathering data, a peril.
[tr. Pound (1933)]

He who learns but does not think, is lost. He who thinks but does not learn is in great danger.
[tr. Waley (1938)]

If one learns from others but does not think, one will be bewildered. If, on the other hand, one thinks but does not learn from others, one will be in peril.
[tr. Lau (1979)]

If one studies but does not think, one is caught in a trap. If one thinks but does not study, one is in peril.
[tr. Dawson (1993)]

To study without thinking is futile. To think without studying is dangerous.
[tr. Leys (1997)]

Learning without thinking is fruitless; thinking without learning is perplexing.
[tr. Huang (1997); additional translations.]

Studying but not thinking, it is confused; Thinking but not studying, it is dangerous.
[tr. Cai/Yu (1998)]

Learning without due reflection leads to perplexity; reflection without learning leads to perilous circumstances.
[tr. Ames/Rosemont (1998)]

If he studies and does not reflect, he will be rigid. If he reflects but does not study, he will be shaky.
[tr. Brooks/Brooks (1998)]

To learn and never think -- that's delusion. But to think and never learn -- that is perilous indeed!
[tr. Hinton (1998)]

If you learn without thinking about what you have learned, you will be lost. If you think without learning, however, you will fall into danger.
[tr. Slingerland (2003)]

Learning without thought is pointless. Thought without learning is dangerous.
[tr. Watson (2007)]

If you learn but do not think, you will be dazed. If you think but do not learn, you will be in danger.
[tr. Chin (2014)]

Learning from books without critical thinking results in confusion. Thinking vacuously without learning from books is perilous.
[tr. Li (2020)]