If you know everything about a simple system, it's easy to predict what it will do. Given the order of cards in a deck, you can "forecast" the next card to appear --- as long as the deck remains unshuffled. Given the quantum-mechanical state of an atom, you can "forecast" the future evolution of its wavefunction --- as long as no outside perturbations induce transitions. Prediction works only for a decoupled system, isolated from the rest of the universe.
But it's a huge leap from a toy problem to even the tiniest real-world situation. Knowing the configuration of a single-cell organism isn't enough to foresee what it will do. Uncertainties cascade; infinitesimal disturbances ripple and grow. External forces like the absorption and emission of infrared photons (heat!) randomize the details of the system. Gross properties remain unchanged, but critical fine structures shift and flow ... and forecasts fail.
Thursday, July 20, 2000 at 17:14:19 (EDT) = Datetag20000720