A pathetic fallacy attributes human emotions and thought processes to something other than a human — an object or an animal, which is known as anthropomorphizing. The concept can apply to the construction of a scientific hypothesis or to the description of a physical phenomenon. It can also refer to a rhetorical technique that makes a connection between dissimilar subjects — the emotion of anger and and the sea, for example — or personifies inanimate objects. "Pathetic fallacy" is not a derogatory term. "Pathetic" has the same root as "empathy," which means to physically impart emotions to another.
The concept was originally described by John Ruskin in the 19th century. In a critique of the then-prevalent practice of personification of the natural world, Ruskin denounced what he considered an abrogation of truth in the pursuit of over-imaginative artistic expression. He coined the phrase specifically to describe a mistaken identification of animate with inanimate elements. As the Romantic era progressed, a tendency to describe and consider the world in strictly empathetic terms became popular among those engaged in intellectual pursuits.
When discussing scientific description, a pathetic fallacy indicates a failure of logic in the construction of a theory or theoretical framework. Though it is less relevant in the discussion of contemporary scientific practice, much of the science of previous eras vacillated about the merits of imparting human emotional and intellectual capacity to non-human objects. Any explanation that ascribes motive to an object engages in a pathetic fallacy.
In literary terms, a pathetic fallacy is useful insofar as it helps to establish metaphoric relationships between objects or abstract concepts that are not easily established. For example, in Shakespeare's Macbeth, there are numerous instances in which nature is anthropomorphized — a "feverous" Earth or an "unruly" night — to construct a theme that shows disapproval of Macbeth's taking the throne of Scotland. This disapproval is made clear by the comparison of what is natural with the consequences of subverting nature. Obviously, night itself cannot be "unruly." Shakespeare extends the pathetic fallacy further with the prophecy that Macbeth will only be killed when a forest walks up to his castle and a man who was not born of a woman arrives to slay him.
The pathetic fallacy can also simplify the discussion of abstract concepts. For example, an instructor may wish to say "an object in motion wants to stay in motion, until acted upon by an external force." Obviously, the object does not "want" anything. It does not have desires or motives of any kind. But such a fallacy can help students understand a concept with which they are unfamiliar.