in practice, but if the starting conditions were known in enough detail, the outcomes of events in such systems could be predicted. Nearly all electronic computers in use today are based on theoretical von Neumann computers or Turing machines, i.e.: they are devices that perform one small, deterministic step at a time. If all inputs are specified, the computer will always produce a particular output which is calculated deterministically. Computer scientists also study other models of computation including parallel computers (more than one deterministic step at a time), and quantum computers (which are based on non-deterministic quantum mechanical models). Computer systems or programs are often described as non-deterministic if their behavior depends on factors that cannot be predicted or reliably reproduced, such as the time of day or the speed at which the user enters data at the keyboard. This, however, is a somewhat different usage of the term. Behaviorism, an approach to psychology based on the proposition that behavior can be researched scientifically without recourse to inner mental states, is usually considered to be deterministic and opposed to free will. Non-deterministic systems Events without natural causes cannot be part of a deterministic system. Whether such events actually occur is a matter of philosophical and scientific debate – however, possible uncaused events include: Random Quantum events Quantum physics holds that certain events such as radioactive decay and movement of particles are completely random when taken at the level of single atoms or smaller. Schrödinger's cat is a famous thought experiment in which a cat's survival cannot be determined theoretically before the experiment is done. For almost all everyday non-microscopic occurrences, however, the probability of such random events is extremely close to zero, and can be approximated to almost certainty with statistics using the correspondence principle. The philosophical consequences of quantum physics were once considered by many (including Albert Einstein) to be a major problem for the scientific method which traditionally used a strong version of scientific determinism (see Philosophy of science). Systems with controversial classification Some systems are particularly difficult to classify as deterministic or not, and have generated much philosophical debate. The major example would be human minds, and possibly animal minds too. Can people have free will if their minds are truly deterministic? Conversely, when deterministic computers are said to exhibit artificial intelligence, how are their minds similar to ours? The entire universe The larger the deterministic system,