Much has been said over the years about what makes humans different from other animals. Perennial favorites include large brains, upright bipedal posture, dexterity and tool use, binocular vision, an articulate vocal tract, symbolic language, and the ability to reason and problem-solve. All of these are relevant, but I would suggest that none of them quite hit the mark. Birds, apes, dogs, cats, and raccoons have collectively shown most of these traits in non-trivial ways.
I would suggest that humans stand out in a rather different way, enabled by all of the factors listed above, but not limited to them. I would suggest that humans are prodigious hallucinators.
The story of hallucination may begin with memory. A wide range of mammals have the ability to store and retrieve several kinds of memory, including working, short-term, and long-term forms. Humans trump all other animals studied. Working memory is perhaps the most important of these, and corresponds to parts of the brain that have expanded most dramatically first in primates, then great apes, then in humans.
Working memory is perhaps the most immediate form of memory, in that it involves not merely storage and retrieval, but the sustained maintenance of activity on populations of neurons during cognitive tasks. It appears pivotal in the manipulation of mental models to predict and consider what might happen in the future, including understanding the effects of various behavioral choices.
Through such 'hallucination,' or understanding of what is not currently present to the senses, humans can consider a wide range of causes and effects, and guide their search through this space using a sense of value. What is desirable or undesirable? How can I figure out how to get what I want?
Other animals can do this, but we humans appear to be capable of this in the extreme.