When Macbeth refers to "supernatural soliciting" in Act One of the play named after him, he's referring to the predictions made by the witches. Basically, Macbeth is thinking aloud, trying to figure out if the witches are good or evil, whether they are trying to help him or doom him.
Macbeth reasons that the witches cannot be evil or wrong or lying because they have given him an "earnest" (a down payment) on the second prediction (that he will be king), since the first prediction (that he will be Thane of Cawdor) has come true. Macbeth reasons that the witches must be speaking accurately and truthfully to him, rather than trying to "play" him. He assumes that because the first prediction came true, the witches must be good, not evil.
At the same time, Macbeth reasons that if the witches are good and are speaking in a clear, straightforward manner, then why have their predictions led him to think the unthinkable--that he should kill the king in order to get the throne for himself. Something good cannot lead to something so evil.
Thus, Macbeth reasons that the witches can't be evil or good. He is at this point undecided about them and their predictions.
Macbeth's reasoning, of course, is faulty here. He assumes that a prediction that comes true must come from a "good" source. Obviously, that is not the case. He also fails to understand that he begins to think about assassinating the king because of his tremendous ambition. His thoughts do not necessarily reflect on the witches. They reflect on him. A different man, his foil, Banquo, for instance, can hear predictions similar to those concerning Macbeth, without immediately thinking of murder to make those predictions come true in a hurry. Banquo would love to see his heirs rule Scotland, but he is content to wait and let fate take its course.