You may know me. I'm your constant companion. I'm your greatest helper; I'm your heaviest burden. I will push you onward or drag you down to failure. I am at your command. Half the tasks you do might as well be turned over to me.
I'm able to do them quickly, and I'm able to do them the same every time, If that's what you want.
I'm easily managed; all you've got to do is be firm with me. Show me exactly how you want it done; after a few lessons I'll do it automatically. I am the servant of all great men and women; of course, I'm the servant of all the failures as well. I've made all the winners who have ever lived. And, I've made all the losers too.
But I work with all the precision of a marvelous computer, With the intelligence of a human being. You may run me for profit, or you may run me to ruin; It makes no difference to me.
Take me. Be easy with me, and I will destroy you. Be firm with me, and I'll put the world at your feet. Who am I?