The word "liberty" and the word "freedom" are strictly synonymous and interchangeable. The word "freedom" carries an adjective that can be used as a modifier. The word "liberty" does not lend itself to such usage.
The Institute for Philosophical Research spent ten years studying the idea of freedom, and published two large volumes on the subject. Liberty is also one of the six great ideas that we studied.
The word "freedom" and with it the word "liberty" cover four major modalities of freedom. These had to be carefully identified in order to discover the authors who discussed and disputed freedom in one or more of these four modalities. In the Institute's work, we discovered two ways in which to identify these modalities of the subject.
The first involved identifying how the freedom or liberty is possessed, either naturally, by acquisition, or circumstantially. Thus, for example, human beings have the freedom of a free will either as a natural endowment, or they do not have it all. They have the moral freedom or liberty of being able to will as they ought as a consequence of moral virtue on their part; and their freedom to act as they please derives from the favorable circumstances under which they live.
The second method of identifying the four freedoms involved a statement about what natural, acquired, and circumstantial freedoms consist in. In the case of natural freedom, it was free choice -- the ability to choose otherwise in every act of free will. We have already seen that moral liberty is the ability to will as one ought to will; and circumstantial freedom is the freedom to do as we please.
We called the circumstantial freedom the freedom of self-realization; the moral liberty, the acquired freedom of self-perfection; and the natural freedom, the freedom of self-determination.
That left only one other freedom to identify, one that really is an aspect of circumstantial freedom; namely, political liberty. A people have political liberty under favorable circumstances when they are governed with their own consent and with a voice in their government.
These four modalities of freedom or liberty do not involve an equivocal use of those words, for we were able to show a third that ties them all together. That thread of meaning can be expressed in the statement that what is common to all four modalities (or certainly the major three, if not that aspect of circumstantial freedom which is political liberty) lies in the power of the self to be dominant.