I am you 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 comand.

Half the things you do you might just as well turn over to me and I will be able to 

do them 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 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.

WHO AM I?