As a social science, economics attempts to use the principles and methods of science to explain economic behaviour. This involves making positive statements about the economic world.
Positive statements are those that can be verified, and are factual, such as:
‘.. House prices have fallen by 15% over the last year…’
In contrast, normative statements are based on opinion and value judgement. Statements suggesting that something ‘ought to’ happen, or that something is ‘unfair’, are normative because they are matters of opinion.
For example, ‘..the recent fall in house prices is unfair to the rich..’.
This statement cannot be tested because it not based on anything testable. If there is an agreed definition of fairness, and it can be measured, then it might be possible to test the effect of the change in house prices on the degree of fairness experienced by a certain identifiable group of people defined as rich. Therefore, this statement is normative, impossible to verify, and based on opinion rather than fact.
Economics is a social science, and, unlike the physical sciences, cannot engage in controlled experimentation to demonstrate how variables are connected.
In the real world, economic variables such as price and income, are constantly changing, and this creates a problem in demonstrating the relationship between variables. For example, a fall in price is likely to lead to a rise in consumer demand if we assume nothing else changes.
Of course, for independent reasons, income could also fall while demand does not rise. The fall in price could have been counteracted by a fall in income. The ceteris paribus rule, that all other things remain the same, is used whenever attempting to demonstrate the link between economic variables.