In its simplest form:
Price effect = income effect + substitution effect
Slutsky asserted in 1915 that demand theory is based on the concept of ordinal utility.
This idea was developed by Hicks who separated the consumer's reaction to a price change into income and substitution effects.
E Slutsky, 'On the Theory of the Budget of the Consumer', Readings in Price Theory, K E Bould-ing and G J Stigler, eds (1953)