Human energy can be broken down into two categories: 

1. muscular energy, and
2. mental energy

Man is, physically speaking, a highly UN-specialized creature.

Unlike other forms of life, man cannot rely upon his biological makeup to sustain his life.

He cannot for example, grow himself a fur coat to protect him from the cold, nor cut down trees with his teeth.  Nor is he able to outrun game animals or see great distances.

Man is able to employ only a limited degree of physical energy to aid him in his survivial.  The opposable thumb and his ability to stand erect are two physical traits which have been of great service, but in order to account for the great success man has had in surviving and thriving we must look for another attribute.

That attribute is man's mental energy - his intellect.

The factor that most clearly distinguishes man from other forms of life is that man relies upon a rational (as opposed to instinctive) faculty.

Specifically, man is able to identify problems, and to imagine and propose specific solutions to those problems.

With this ability, man has been able to devise tools that have been used to augment his human energy, in order to give him an advantage over others in his environment, and the environment itself.