Niven's Law is also a term given to the converse of Clarke's third law, so Niven's Law reads: "Any sufficiently rigorously defined magic is indistinguishable from technology."
If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.
Drawn from Known Space: The Future Worlds of Larry Niven
1.
a.Never throw crap at an armed man.
b.Never stand next to someone who is throwing crap at an armed man.
2.Never fire a laser at a mirror.
3.Mother Nature doesn't care if you're having fun.
4.F × S = k. The product of Freedom and Security is a constant. To gain more freedom of thought and/or action, you must give up some security, and vice versa
5.Psi and/or magical powers, if real, are nearly useless.
6.It is easier to destroy than create.
7.Any fool can predict the past.
8.History never repeats itself.
9.Ethics change with technology.
10.Anarchy is the least stable of social structures. It falls apart at a touch.
11.There is a time and place for tact.
12.The ways of being human are bounded but infinite.
13.The world's dullest subjects, in order:
a.Somebody else's diet.
b.How to make money for a worthy cause.
c.Special Interest Liberation.
14.The only universal message in science fiction: There exist minds that think as well as you do, but differently.
Niven's corollary: The gene-tampered turkey you're talking to isn't necessarily one of them.
15.Fuzzy Pink Niven's Law: Never waste calories.
16.There is no cause so right that one cannot find a fool following it.
in variant form in Fallen Angels as "Niven's Law: No cause is so noble that it won't attract fuggheads."[3]
17.No technique works if it isn't used.
18.Not responsible for advice not taken.
19.Old age is not for sissies.