Schrodinger's cat, the most famous thought experiment in physics, was created by Erwin Schrodinger, who was uncomfortable with the implications of his discoveries in the field of quantum mechanics. Tiny entities like electrons or atoms are described by Schrodinger wave functions. In contrast, the wave function description is not useful for big things like cats because even the slightest interaction between the cat and the surrounding environment obliterates its wave function. For example, the purr of an alive cat may reach the outside world, signaling the cat's continued health to any observer who cares to listen. Even supposing it were somehow possible to completely contain all sounds emanating from the chamber, still there would be a slight but detectable change in the gravitational field as the cat moves about.
Any large disturbance leaves traces that wash out subtle quantum effects.
Quantum theory is not appropriate for cats or most other things larger than atoms, such as the human brain. Some have been struck, however, by an apparent parallel between how the mind works and how quantum wave functions collapse. When we make a sudden decision it seems that only one choice, out of multitude of subconscious possibilities, takes conscious form. Likewise, when an observer checks on the health of Schrodinger's quantum cat, it seems as if nature must choose between a dead cat or a live cat. Mathematician Roger Penrose has gone further to propose that the brain really is a quantum computer.