The human beings do not find in themselves the totality of elements necessary for their existence and for their development. It is in the outside environment, in the nature, that they have to look for necessarily these elements. But, these last ones do not exist forcing in unlimited quantities there, they are more or less rare (uncommon). To satisfy an economic need, it is to fight against the rarity. Now, the fight imposes an effort which the economists appoint under the term of cost.
- Lecturer: UWIMANA NDIYAYE Innocent