In science 'fact' can only mean 'confirmed to such a degree that it would be perverse to withhold provisional assent.' I suppose that apples might start to rise tomorrow but the possibility does not merit equal time in physics classrooms.
Optimism is not inherently a superior way of viewing the world. Certainly doctors will say it might be better for one's physical health to be an optimist. But morally speaking it may not be appropriate in certain circumstances.
