Explain the law of variable proportion

The following three people explained the Law of Variable Proportions:

1. “As the proportion of the factor in a combination of factors is increased after a point, first, the marginal and then the regular product of that factor will diminish.” Benham
2. “An increase in some inputs relative to other fixed inputs will in a given state of technology cause output to increase, but after a point, the extra output resulting from the same additions of extra inputs will become less and less.” Samuelson
3. “The law of variable proportion states that if the inputs of one resource are increased by equal increment per unit of time while the inputs of other resources are held constant, total output will increase, but beyond some point, the resulting output becomes higher will become smaller and smaller.” Leftwitch