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 you do might just as well be turned 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 people 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 person. You may 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! What am I? A habit.