“Happiness depends more upon the internal frame of a person’s own mind, than on the externals in the world.”

The Robert Heinlein Interview (1973)
Context: I would say that my position is not too far from that of Ayn Rand's; that I would like to see government reduced to no more than internal police and courts, external armed forces — with the other matters handled otherwise. I'm sick of the way the government sticks its nose into everything, now.

Source: 2010s, Waking Up (2014), p. 204

Who Is a Free Man. What Is Freedom? http://parentingforeveryone.com/freeman/
Chelovek Svobodny (Free Man) (1994)

An interpretative gloss of Aristotle's position in Nicomachean Ethics book 1 section 9, tacitly inserted by J. A. K. Thomson in his English translation The Ethics of Aristotle (1955). The original Greek at Book I 1099b.29 http://perseus.uchicago.edu/perseus-cgi/citequery3.pl?dbname=GreekFeb2011&getid=0&query=Arist.%20Eth.%20Nic.%201099b.25, reads ὁμολογούμενα δὲ ταῦτ’ ἂν εἴη καὶ τοῖς ἐν ἀρχῇ, which W. D. Ross translates fairly literally as [a]nd this will be found to agree with what we said at the outset. Thomson's much freer translation renders the same passage thus: [t]he conclusion that happiness depends upon ourselves is in harmony with what I said in the first of these lectures; the words "that happiness depends upon ourselves" were added by Thomson to clarify what "the conclusion" is, but they do not appear in the original Greek of Aristotle. Rackham's earlier English translation added a similar gloss, but averted confusion by confining it to a footnote.
Disputed
Variant: Happiness depends upon ourselves
Source: See http://www.mikrosapoplous.gr/aristotle/nicom1b.htm#I9 for the original Greek and Ross's translation; Thomson's translation can be viewed on Google Books https://books.google.com/books?id=9SFrNWmO654C&dq=%22happiness+depends+upon+ourselves%22+aristotle&focus=searchwithinvolume&q=%22happiness+depends+upon+ourselves%22+.
Source: Rackham's translation of this passage is available here http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0054%3Abook%3D1%3Achapter%3D9%3Asection%3D8

The Secret Daily Teachings

Fernbank, London, England October, 1971
1970s
Variant: If you want external happiness, it can be an elusive desire. Internal happiness needs only to be revealed. It is not elusive because it is within you. It is your treasure. If you take someone else's treasure, it is stealing, but if you turn to your own, it is not. Happiness is your own treasure because it lies within you.

Romancing Opiates: Pharmacological Lies and the Addiction Bureaucracy (2006)