But how do we tell if an action is an action of a good will? Well, we text the maxim, or the personal rule according to which a person was acting. Another name for a maxim is a principle of volition. The idea is this. For every action, there is some personal rule that a person is following. In order to test if this maxim is good, we must use a certain kind of test for this rule.
The test we use is the Categorical Imperative. The categorical imperative states that one should only act according to rules that can be made into universal law. For some rules, it is logically impossible to make them a universal law. For example, if you wanted to make "It is ok to tell a lie whenever you want" a universal rule, this would be logically impossible. Lying means telling someone falsehoods under the pretense that they are truths. In other words, lying means telling someone something false under the assumption that they will believe your words anyway. If everyone lied whenever they wanted to, then nobody would ever believe anyone. Lying would be impossible because lying requires that the person to whom you are lying believes your lies.
Here is another example. Say I am in a hurry at Starbucks and I want to cut in line. So I act according to the maxim, "It is ok to cut in line when I want". If this rule became a universal law and everyone cut in line whenever they wanted, then lines would cease to exist. If the rule became universalized, then it would become logically impossible to follow that rule. The point is not just that it would be an impractical rule to be universalized. Rather, it would be logically impossible to universalize this rule because if everyone followed it, then following it would become impossible!