How can you interpret Thomas Jefferson's words "all men are created equal" in a racial sense, when he said this?

"The improvement of the blacks in body and mind, in the first instance of their mixture with whites, has been observed by every one, and proves that their inferiority is not the effect merely of their condition of life. We know that among the Romand, about the Agustan age especially, the condition of their slaves was much more deplorable than that of blacks on the continent of America. Yet notwithstanding these and other discouraging circumstances among the Romans, their slaves were often their rarest artists. They excelled too in science, insomuch as to be usually employed as tutors to their masters' children. Epictetus, Terence, and Phaedrus, were slaves. But they were of the race of whites. It is not their condition then, but nature, which has produced the distinction. Among the Romans emancipation required but one effort. The slave, when made free, might mix with, without staining the blood of his master. But with us a second is necessary, unknown to history. When freed, he is to be removed beyond the reach of mixture."-Thomas Jefferson

http://www.let.rug.nl/usa/presidents/thomas-jeffer...