A maxim is simply a moral rule or principle, which can be considered objective or subjective, and dependent on one's philosophy. A maxim is often pedagogical and motivates specific actions. The Oxford Dictionary of Philosophy defines it as:
Generally any simple and memorable rule or guide for living; for example, 'neither a borrower nor a lender be'. Tennyson speaks of 'a little hoard of maxims preaching down a daughter's heart (Locksley Hall), and maxims have generally been associated with a 'folksy' or 'copy-book' approach to morality.
In deontological ethics, mainly in Kantian ethics, maxims are understood as subjective principles of action. A maxim is thought to be part of an agent's thought process for every rational action, indicating in its standard form: (1) the action, or type of action; (2) the conditions under which it is to be done; and (3) the end or purpose to be achieved by the action, or the motive. The maxim of an action is often referred to as the agent's intention. In Kantian ethics, the categorical imperative provides a test on maxims for determining whether the actions they refer to are right, wrong, or permissible.
The categorical imperative is stated canonically as: "Act only according to that maxim whereby you can, at the same time, will that it should become a universal law."
In his Critique of Practical Reason, Immanuel Kant provided the following example of a maxim and of how to apply the test of the categorical imperative:
I have, for example, made it my maxim to increase my wealth by any safe means. Now I have a deposit in my hands, the owner of which has died and left no record of it. . . . I therefore apply the maxim to the present case and ask whether it could indeed take the form of a law, and consequently whether I could through my maxim at the same time give such a law as this: that everyone may deny a deposit which no one can prove has been made. I at once become aware that such a principle, as a law, would annihilate itself since it would bring it about that there would be no deposits at all.