Prof. Robbins has defined Economics as a science of choice. According to him economics is the science which studies human behaviour in relation with unlimited wants of individual and limited or scarce resources. That's how an individual chooses a combination of goods among different goods to satisfy his wants along with limited amount of money.
Human wants are unlimited and no one can satisfy his all wants fully because resources to satisfy them are limited. As an individual satisfies one want at a particular time, another want crops up and a human being is interwoven into the web of different wants having never ending circle of wants every one tries to make efforts to satisfy his/her wants with limited means and chooses only few wants among various.