“It isn't the desire to abide by the law that makes everyone behave as society requires, but the fear of punishment. Each one of us carries a gallows inside us.”

Source: Law and Authority (1886), II
Context: Legislators confounded in one code the two currents of custom of which we have just been speaking, the maxims which represent principles of morality and social union wrought out as a result of life in common, and the mandates which are meant to ensure external existence to inequality.
Customs, absolutely essential to the very being of society, are, in the code, cleverly intermingled with usages imposed by the ruling caste, and both claim equal respect from the crowd. "Do not kill," says the code, and hastens to add, "And pay tithes to the priest." "Do not steal," says the code, and immediately after, "He who refuses to pay taxes, shall have his hand struck off."
Such was law; and it has maintained its two-fold character to this day. Its origin is the desire of the ruling class to give permanence to customs imposed by themselves for their own advantage. Its character is the skillful commingling of customs useful to society, customs which have no need of law to insure respect, with other customs useful only to rulers, injurious to the mass of the people, and maintained only by the fear of punishment.

Source: Discipline and Punish (1977), Chapter Three, The Gentle Way in Punishment

Source: The Storyteller

Speech to the Society of Professional Journalists (11 September 2004)
Context: This "zeal for secrecy" I am talking about — and I have barely touched the surface — adds up to a victory for the terrorists. When they plunged those hijacked planes into the World Trade Center and the Pentagon three years ago this morning, they were out to hijack our Gross National Psychology. If they could fill our psyche with fear — as if the imagination of each one of us were Afghanistan and they were the Taliban — they could deprive us of the trust and confidence required for a free society to work. They could prevent us from ever again believing in a safe, decent or just world and from working to bring it about. By pillaging and plundering our peace of mind they could panic us into abandoning those unique freedoms — freedom of speech, freedom of the press — that constitute the ability of democracy to self-correct and turn the ship of state before it hits the iceberg.

Source: Aphorisms (1880/1893), p. 82.

Misattributed
Source: Frequently quoted on social media, but appears to be a misquote of Thomas Browne's "We carry within us the wonders we seek without us: there is all Africa and her prodigies in us" in Religio Medici (1643) pt. 1, sect. 15.

The Midwich Cuckoos (1957), ch 18 - p.182 [Eric]

Variant: Books are mirrors: you only see in them what you already have inside you.
Source: The Shadow of the Wind

2008, Youth Day Vigil (19 July 2008)