ceteris paribus

Imagine you want to know whether a new fertiliser helps tomatoes grow. The fair way is to change only the fertiliser and keep everything else the same — same soil, same water, same sunlight — so any difference must be due to the fertiliser, not the weather. The economy cannot be put in a lab like that, so economists make the assumption in their heads instead. They say ceteris paribus: a Latin phrase meaning 'other things being equal'.

Ceteris paribus is the assumption that, while we examine the effect of one variable, all other influences are held fixed. It is how economists isolate a single cause. When a textbook says 'if the price of coffee rises, ceteris paribus, people buy less coffee', it means: holding incomes, tastes, the prices of tea, and everything else constant, the price change alone pushes quantity down. Without this clause the statement would be unreliable, because in the real world a price rise might coincide with rising incomes that lift demand, muddying the picture.

Ceteris paribus is indispensable for clear thinking, but it is also where economic predictions can go wrong, because in reality other things are seldom equal — many forces move at once. A demand 'law' that holds all else constant can seem to fail in raw data when incomes, fashions, and rival prices are all shifting too. The phrase is a reminder, not a magic spell: it tells you what a relationship would look like in isolation, and warns you to check whether the held-constant assumptions actually held before trusting the conclusion.