The essence of the law of diminishing returns is

The essence of the law of diminishing returns is negative marginal production.

Explanation:

1. Law Of Diminishing Returns states that as one variable input is increased, with all others remaining fixed, a point will be reached beyond which the marginal physical product of the variable factor will begin to decrease.
2. Still, if additional variable input is added, it will lead to negative marginal production. 
3. For example, if a factory employs workers to manufacture its products, at some point, the company will operate at an optimal level; with all other production factors constant, adding additional workers beyond this optimal level will result in less efficient operations as the size of the factory and equipment will remain as it is but workers will increase resulting in the chaos which will reduce efficiency.