What was the philosophy of Thomas Jefferson?
There are at least two ways to answer this.
On the one level, you can say that Jefferson's philosophy was similar to that of John Locke. This can be seen in the Declaration of Independence. Jefferson believed that government had to be by the consent of the people and that the government had to protect the people's rights.
On another level, you can identify Jefferson as a democrat (as opposed to someone who believed in a republic). Jefferson wanted the common people to be involved in government and for the government to adhere closely to the will of the people. He wanted a relatively egalitarian society in which essentially everyone was more or less equal. He felt that this was the best way to preserve democracy and, thereby, liberty.