“Bishop Jeremy Taylor is clear, that men will find it impossible to do anything greatly good, unless they cut off all superfluous company and visits.”

As quoted in "The Vindication of Edwin Land" in Forbes magazine, Vol. 139 (4 May 1987) p. 83; this was later humorously altered:
Don't do anything that someone else can do. Don't undertake a project unless it is manifestly important and nearly impossible.
Edwin Ladd, Department of Astronomy, Harvard University (August, 1990), as quoted by Lincoln J. Greenhill at Harvard University http://www.cfa.harvard.edu/~lincoln/

As quoted in "The Vindication of Edwin Land" in Forbes magazine, Vol. 139 (4 May 1987) p. 83; this was later humorously altered:
Context: My motto is very personal and may not fit anyone else or any other company. It is: Don't do anything that someone else can do. Don't undertake a project unless it is manifestly important and nearly impossible.

Enoch Powell, Joseph Chamberlain (Thames and Hudson, 1977), p. 151
1970s

Man a Machine (1747)

Aurea Dicta VI, p. 4.
The Rod, the Root, and the Flower (1895)

Carpet Remnant World

As quoted in "WHAT HENDRIX NEVER SAID : They Don't Want to Know What He Really Said and Demand a Slacker Fantasy Instead" (22 March 2010) by Michael Fairchild, at rockprophecy.com; the author does not provide any sourcing for this statement, beyond his assertion that his authority and expertise should be trusted because: "I was assigned to compile all known quotes of Jimi Hendrix and edit this text into the "autobiography" of Jimi, in his own words. That book has been censored/suppressed by all world publishers for two decades now, but it's where I became familiar with Jimi's unique syntax, vernacular, and habits of thought."
Disputed
Context: There's so many tight-lipped ideas and laws around, and people put themselves in uniforms so tightly, that it's almost impossible to break out of that. Subconsciously, what these people are doing, they're killin' off all these little flashes they have, cutting off the idea of wanting to understand. They forgot, didn't believe, or just snuffed the feelings or thoughts off to continue with their crazy soul. They don't have the patience to really check out what's happenin' through music, theater and science. It’s like a spaceship. If a spaceship came down and you know nothin’ about it, the first thing you’re going to think about is shooting it. In other words, you get negative in the first place, which is not really the natural way of thinking. It’s like shooting at a flying saucer as it tries to land without giving the occupants a chance to identify themselves.

Golden Sayings of Epictetus
Context: Let silence be your general rule; or say only what is necessary and in few words. We shall, however, when occasion demands, enter into discourse sparingly, avoiding such common topics as gladiators, horse-races, athletes; and the perpetual talk about food and drink. Above all avoid speaking of persons, either in the way of praise or blame, or comparison. If you can, win over the conversation of your company to what it should be by your own. But if you should find yourself cut off without escape among strangers and aliens, be silent. (164).

Tripadis