Full Idea
When we say something is 'physically necessary' we can replace it with 'law of nature'.
Gist of Idea
A 'law of nature' is just something which is physically necessary
Source
Roderick Chisholm (Person and Object [1976], 2.2)
Book Reference
Chisholm,Roderick: 'Person and Object' [Open Court 1976], p.58
A Reaction
[plucked out of context even more than usual!] This is illuminating about what contemporary philosophers (such as Armstrong) seem to mean by a law of nature. It is not some grand equation, but a small local necessary connection.