Thomas Aquinas — "Ignorance is the cause of all evil."
Ignorance is the cause of all evil.
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More from Thomas Aquinas
"The intellect is the highest power of the soul."
"The intellect is perfected by truth, and the will by good."
"The virtue of chastity is a part of temperance."
"Strictly speaking, woman is a monster of nature."
"It is impossible for any created intellect to comprehend God."
Details
Often attributed, but a more precise phrasing in Aquinas is that ignorance can be a cause of sin, not all evil.
Date: c. 1265-1274
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Understanding this quote
What it means
When people do wrong, it's because they lack true understanding of what is good. Evil isn't a separate force — it's what happens when knowledge fails. If someone genuinely understood the consequences of their actions and the nature of the good, they would choose differently. Ignorance — of morality, of consequences, of truth — is what allows harmful choices to feel reasonable or justified.
Relevance to Thomas Aquinas
Aquinas spent his life synthesizing reason and faith, arguing that human intellect, properly guided, leads to God and moral goodness. His Summa Theologica was built on the belief that clear understanding eliminates error. He saw the will as dependent on the intellect: a person cannot genuinely desire evil if they truly know the good. His Dominican order was founded precisely to combat heresy through preaching and education.
The era
Medieval Europe in the 1200s was gripped by religious wars, widespread illiteracy, and the Inquisition targeting heresy. Universities were just forming, and Aristotle's rediscovered works were reshaping scholarship. The Church held that sin and social disorder stemmed from straying from God's truth. In this climate, declaring ignorance the root of evil was both a philosophical position and a practical call for expanding theological education as society's moral cure.
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