Who am I?

Quoting the following excerpt from a book I'm reading...

I am your constant companion. I am your greatest helper or heaviest burden. I will push you onward or drag you down to failure. I am completely at your command. Half the things I do you might just as well turn over to me and I will be able to do them quickly and correctly.

I am easily managed - you must merely be firm with me. Show me exactly how you want something done and after a few lessons I will do it automatically. I am the servant of all great individuals and, alas, of all failures, as well. Those who are great, I have made great. Those who are failures, I have made failures.

I am not a machine, though I work with all the precision of a machine plus the intelligence of a human. You may run me for a profit or run me for ruin - it makes no difference to me.

Take me, train me, be firm with me, and I will place the world at your feet. Be easy with me and I will destroy you.

Any guesses? =)