There is no difference. Negation in English is accomplished by adding the word "not" at the end of the verb; the negation of "He does" is "He does not." Also in English, the addition of the verb "to do" to another verb does not change its meaning. It is sometimes used for emphasis: "She knows" means the same as "She DOES know."; the latter is only more emphatic.Habitually, English-speakers add the verb "to do" to a verb when they want to negate it. Thus instead of "He drives a truck" they say "He does not drive a truck." "He drives not a truck" would not be colloquial, even though it is less unnecessarily long-winded."Does not" is often contracted into "Doesn't" and "Do not" into "Don't". Thus "I eat poutine" is expressed in the negative as "I don't eat poutine."So, as a result. "He seems not to have put on his pants this morning." and "He doesn't seem to have put on his pants this morning." mean exactly the same thing.