The phrase "knowledge is power" is often attributed to Francis Bacon, from his Meditationes Sacrae (1597).
Thomas Jefferson used the phrase in his correspondence on at least four occasions, each time in connection with the establishment of a state university in Virginia.
In an 1817 letter to George Ticknor, Jefferson equated knowledge with power, safety, and happiness:
[T]his last establishment [a state university] will probably be within a mile of Charlottesville, and four from Monticello, if the system should be adopted at all by our legislature who meet within a week from this time. my hopes however are kept in check by the ordinary character of our state legislatures, the members of which do not generally possess information enough to percieve the important truths, that knolege is power, that knolege is safety, and that knolege is happiness.
In two 1820 letters to Joseph Cabell, Jefferson again emphasized the importance of knowledge:
Kentucky, our daughter, planted since Virginia was a distinguished state, has an University, with 14. professors & upwards of 200 students. ... all the states but our own are sensible that knolege is power.
[I]t is unquestionable that [Virginia] has more influence in our confederacy than any other state in it. whence this ascendancy? from her attention to education unquestionably. there can be no stronger proof that knolege is power, and that ignorance is weakness.
Continuing to write on the subject of a state university, Jefferson referred to the power of knowledge in an 1821 letter to John Taylor:
[Northeastern] seminaries are no longer proper for Southern or Western students. the signs of the times admonish us to call them home. if knolege is power we should look to it's advancement at home, where no resource of power will be unwanting.
931 Thomas Jefferson Parkway
Charlottesville, VA 22902