Arthur Samuel wrote an early (1959) program that played Checkers at championship level. This program had several interesting features:

1. The program learned its heuristic function from experience playing humans and itself. The heuristic function was a weighted average of several features (e.g. piece advantage, mobility, control of the center). Coefficients of features were ``rewarded'' or ``punished'' depending on whether the program won or lost.

2. If the program wins (or loses), it is difficult to determine which move(s) were responsible and should be rewarded or punished. This is called the credit assignment problem .

3. Low-pass filtering of coefficients was used to smooth out the effects of unjust rewards and punishments.

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