Sir

I am very sensible of the honour you propose to me of becoming a member of the society for the abolition of the slave trade. You know that nobody wishes more ardently to see an abolition not only of the trade but of the condition of slavery: and certainly nobody will be more willing to encounter every sacrifice for that object. But the influence and information of the friends to this proposition in France will be far above the need of my association. I am here as a public servant; and those whom I serve having never yet been able to give their voice against this practice, it is decent for me to avoid too public a demonstration of my wishes to see it abolished. Without serving the cause here, it might render me less able to serve it beyond the water. I trust you will be sensible of the prudence of those motives therefore which govern my conduct on this occasion, and be assured of my wishes for the success of your undertaking and the sentiments of esteem and respect with which I have the honour to be Sir your most obedt. humble servt.,

Th: Jefferson

Letter from Jefferson to Brissot de Warville, 11 February 1788

Jefferson owned a little over 600 people through the course of his life, usually 150-200 at a time. A large group (about 125) was recieved after the death of his father in law, John Wayles, in May of 1773. This included the children of Elizabeth Hemings, who was the concubine of Wayles after his third wife died, including a young daughter named Sally (and we could talk about the Jefferson-Hemings relation for a day or two by itself). Jefferson would remain a participant in the practice of slavery until his own death, July 4th 1826, at which point the majority of his humans in bondage were sold, with some kept by family and a small group - all Hemingses - freed.

When the Articles of Confederation were passed, we had a State led nation in that the real power - executive authority and taxation - resided in the individual states and not the American government. How, then, does one stop slavery in the new nation? By stopping it in the state. In 1778, Jefferson tried to do just that. A bill governing slavery was proposed, and it was felt that the road to manumission was through amendment of that bill;

The bill on the subject of slaves was a mere digest of the existing laws respecting them, without any intimation of a plan for a future and general emancipation. It was thought better that this should be kept back, and attempted only by way of amendment, whenever the bill should be brought on. The principles of the amendment, however, were agreed on, that is to say, the freedom of all born after a certain day, and deportation at a proper age. But it was found that the public mind would not yet bear the proposition.

He also wrote of the event;

[T]his subject was not acted on finally until the year [17]78. when I brought in a bill to prevent their further importation. This passed without opposition, and stopped the increase of the evil by importation, leaving to future efforts its final eradication.

His idea was gradual emancipation followed by deportation;

The bill reported by the revisers does not itself contain this proposition; but an amendment containing it was prepared, to be offered to the legislature whenever the bill should be taken up, and further directing, that they should continue with their parents to a certain age, then be brought up, at the public expence, to tillage, arts, or sciences, according to their geniusses, till the females should be eighteen, and the males twenty-one years of age, when they should be colonized to such place as the circumstances of the time should render most proper, sending them out with arms, implements of houshold and of the handicraft arts, seeds, pairs of the useful domestic animals, &c. to declare them a free and independant people, and extend to them our alliance and protection, till they shall have acquired strength; and to send vessels at the same time to other parts of the world for an equal number of white inhabitants; to induce whom to migrate hither, proper encouragements were to be proposed.

In 1784 he wrote that slavery should not be permitted in the newly obtained Northwest (Ohio) which was acted upon when the Northwest Ordinance became a law. And in his Notes on the State of Virginia from 1782 he wrote;

Indeed I tremble for my country when I reflect that God is just; that his justice cannot sleep forever.

He realized the public wasn't ready for emancipation and stopped his efforts to publicly champion it. Looking at the 1788 letter in France at the top, we get a good idea of his thoughts on why - it would reduce his effectiveness as a Virginia politician to beat the same drum. Many point to this as a sign he simply gave up, but others point to quotations throughout the rest of his life that show he always felt emancipation and seperation was the right thing to do;

Nobody wishes more than I do to see such proofs as you exhibit, that nature has given to our black brethren, talents equal to those of the other colors of men, and that the appearance of a want of them is owing merely to the degraded condition of their existence. Jefferson to Banneker, August 1791

There is nothing I would not sacrifice to a practicable plan of abolishing every vestige of this moral and political depravity. Thomas Jefferson to Thomas Cooper, September 1814

It was apparently beyond his power to change. That doesn't give him a pass by any means, but he did take steps to do what he felt was right in the first half of his political career. Those efforts and public acts stopped, however, and it is also notable that this was not very Jefferson-like, he didn't fail to see things through. The man spent 40 years, literally, building his home. He had levelled a mountaintop to do it. Before that started, he developed a contingency plan for blasting bedrock to level it, should they hit it and the need arise. He planned, acted, and accomplished. Except here, where he seemed overwhelmed by this venture of enforcing his words with action. So that's a notable thing that's a bit odd when we look at the larger question, "Who was Thomas Jefferson?"

It would appear that he did not change his mind. He felt blacks were inferior to whites, and believed it to be genetic, but was open to the idea he was wrong and it was the lifestyle of oppression that is slavery preventing their appearing equal. He cited things like how they would stay up past midnight, singing and hanging out, then be up for a hard long day at sunrise, apparently needing very little sleep (unlike, of course, white Americans). He didnt realize that their only "free" time (poor choice of words, I know) was a day or half day on Sunday and at night. So of course they're going to spend some time retaining their individuality as best they can, and that's easy to see from our perspective, but to Jefferson that made them different. So he uses examples of cultural differences or results of bondage as his science, and accordingly he was wrong about his conclusions. But he wasn't alone in thinking them as his logic was fairly common belief in America at that time. He saw them as real and equal in deserving freedom, but as a truly distinct and seperate race of mankind from whites and felt that, coupled with long held animosity for the most atrocious treatment, would lead to a race war in which one side would be eliminated, and he feared it may not be the blacks losing to the whites, which certainly fueled his desire not only for emancipation but for the deportation to an independent colony thereafter.

Jefferson and his Time, Dumas Malone, Vol I: Jefferson the Virginian, Vol II: Jefferson and the Rights of Man, Vol VI: The Sage of Monticello

Thomas Jefferson: The Art of Power, John Meacham

The Hemingses of Monticello: An American Family, Annette Gordon-Reed

Those Who Labor for My Happiness: Slavery at Thomas Jefferson's Monticello, Lucia Stanton (if I could only cite or recommend one book for more info on your question, this would probably be it).

The Autobiography of Thomas Jefferson