Select the most appropriate one-word substitution for the given group of words.

A well known saying or an established principle of practical wisdom 

Maxim

The most appropriate one-word substitution for "a well-known saying or an established principle of practical wisdom" is maxim.

The other words you have provided do not have the same meaning as maxim.

• Principal refers to the main or most important part of something.
• Law refers to a rule or set of rules that must be obeyed.
• Decree refers to a formal order issued by a government or other authority.