“You can never be too cautious in your prognosis, in the view of the great uncertainty of the course of any disease not long watched, and the many unexpected turns it may take.”

Valedictory Address to medical graduates at Harvard University (10 March 1858), published in The Boston Medical and Surgical Journal Vol. LVIII, No. 8 (25 March 1858), p. 158; this has also been paraphrased "Beware how you take away hope from another human being".
Context: You can never be too cautious in your prognosis, in the view of the great uncertainty of the course of any disease not long watched, and the many unexpected turns it may take.
I think I am not the first to utter the following caution : —
Beware how you take away hope from any human being. Nothing is clearer than that the merciful Creator intends to blind most people as they pass down into the dark valley. Without very good reasons, temporal or spiritual, we should not interfere with his kind arrangements. It is the height of cruelty and the extreme of impertinence to tell your patient he must die, except you are sure that he wishes to know it, or that there is some particular cause for his knowing it. I should be especially unwilling to tell a child that it could not recover; if the theologians think it necessary, let them take the responsibility. God leads it by the hand to the edge of the precipice in happy unconsciousness, and I would not open its eyes to what he wisely conceals.