Man the machine--man the impersonal engine. Whatsoever a man is, is due to his MAKE, and to the INFLUENCES brought to bear upon it by his heredities, his habitat, his associations. He is moved, directed, COMMANDED, by EXTERIOR influences--SOLELY. (What Is Man?, Mark Twain, http://underthesun.cc/Classics/Twain/whatman/Whatisman.htm)
There is some scientific basis for this claim. Studies have shown that both a person's genetic structure and the circumstances to which he or she is subjected have bearing on how a person thinks, feels and acts. Considering this, the actions of the character Macbeth must be evaluated by his personal motivations and the external causes that may have led to them. It is established from the very beginning that Macbeth is ambitious. There can be no doubt about this. A certain level of courage accompanies his ambition as well. As a noble he is an active one, fighting against the rebel hordes and Norwegians in defense of his king, no doubt for the purpose of gaining notoriety and other rewards. This is further illustrated by his gracious acceptance of credit for his deeds. He is a political figure in the highest sense, and show ambition in this way. However, there is no sign of him altering his course of loyal nobleman until outside influences begin to intercede. The people with greatest impact on Macbeth are the witches, his wife and Lady, and King Duncan of Scotland. The witches introduce the idea, King Duncan gives personal motive, and Lady Macbeth helps along the way.
The least influential party in all of this is King Duncan. The conflict between these two is purely circumstantial, but clear enough. Macbeth is, as stated, an ambitious man. The