Under the doctrine of separation of powers, one branch of the government cannot, as a rule, encroach upon or perform the primary function of the other branches. Officials of one branch cannot meddle, interfere with, impose upon or dictate how the other branches should perform their jobs.
Basically, this means that within their respective spheres, each department of government is supreme and have the final say on their actions. The reason for this separation of powers is to avoid arbitrary or tyrannical rule if one body exercises all or most of the governmental powers (Am. Jur. 280).