“Every man, as the Stoics used to say, is first and principally recommended to his own care; and every man is certainly, in every respect, fitter and abler to take care of himself than of any other person. Every man feels his own pleasures and his own pains more sensibly than those of other people. The former are the original sensations; the latter the reflected or sympathetic images of those sensations. The former may be said to be the substance; the latter the shadow.”

The Spirit of Christianity and its Fate (1799)

Source: The Wealth of Nations (1776), Book IV, Chapter IX, p. 749.
Context: Every system which endeavours, either, by extraordinary encouragements, to draw towards a particular species of industry a greater share of the capital of the society than what would naturally go to it; or, by extraordinary restraints, to force from a particular species of industry some share of the capital which would otherwise be employed in it; is in reality subversive of the great purpose which it means to promote. It retards, instead of accelerating, the progress of the society towards real wealth and greatness; and diminishes, instead of increasing, the real value of the annual produce of its land and labour.
All systems either of preference or of restraint, therefore, being thus completely taken away, the obvious and simple system of natural liberty establishes itself of its own accord. Every man, as long as he does not violate the laws of justice, is left perfectly free to pursue his own interest his own way, and to bring both his industry and capital into competition with those of any other man, or order of men. The sovereign is completely discharged from a duty, in the attempting to perform which he must always be exposed to innumerable delusions, and for the proper performance of which no human wisdom or knowledge could ever be sufficient; the duty of superintending the industry of private people, and of directing it towards the employments most suitable to the interest of the society. According to the system of natural liberty, the sovereign has only three duties to attend to; three duties of great importance, indeed, but plain and intelligible to common understandings: first, the duty of protecting the society from the violence and invasion of other independent societies; secondly, the duty of protecting, as far as possible, every member of the society from the injustice or oppression of every other member of it, or the duty of establishing an exact administration of justice; and, thirdly, the duty of erecting and maintaining certain public works and certain public institutions, which it can never be for the interest of any individual, or small number of individuals, to erect and maintain; because the profit could never repay the expence to any individual, or small number of individuals, though it may frequently do much more than repay it to a great society.
The proper performance of those several duties of the sovereign necessarily supposes a certain expence; and this expence again necessarily requires a certain revenue to support it.

Source: Meditations

Section 1, member 3.
The Anatomy of Melancholy (1621), Part III

Remarks on the Republican platform (1860)

The Ayn Rand Column ‘Introducing Objectivism’

Often given as: A mind that is stretched by a new experience can never go back to its old dimensions.
Or: A mind that is stretched by a new idea can never go back to its old dimensions.
Actually by Oliver Wendell Holmes, Senior, from " Autocrat of the Breakfast Table https://books.google.com/books?id=BoQ3AQAAMAAJ&pg=PA502&dq=%22stretched+by+a+new+idea%22&hl=en&sa=X&ved=0ahUKEwidspn60tTJAhVJ1GMKHbt3Bn0Q6AEIHTAA#v=onepage&q=%22stretched%20by%20a%20new%20idea%22&f=false", originally published in The Atlantic, September 1858.
Misattributed

Vol. I, ch. 2.
Vanity Fair (1847–1848)
Context: The world is a looking-glass, and gives back to every man the reflection of his own face. Frown at it, and it will in turn look sourly upon you; laugh at it and with it, and it is a jolly kind companion; and so let all young persons take their choice.