The chief limitation of the human intellect is not sheer shortage of brainpower, but defect of character.  The weakness of our minds is not that we are not smart enough, but that we are not good enough.

Consider how faulty our thinking is about other people because we are too interested in ourselves;

If I say that euthanasia should be illegal because murder violates the law of God, then obviously I suppose that there is a God, that He has a law, that this law ought to be obeyed, that it forbids murder, that euthanasia is murder, and that He requires human authority to back him up on such a point.

"Man is obviously made for thinking.  Therein lies all his dignity and his merit; and his whole duty is to think as he ought.  Now the order of thought is to begin with ourselves, and with our author and our end.”

-- Blaise Pascal, Pensées