Practice is often described as applied science, but real life is far too complex and interactive to be handled by analytical scientific methods. The limitations of usefulness of scientific method in clinical practice result from many factors. The complexity of the large number of ill-defined variables at many levels of the problem. Scientific method focuses on one variable at a time across a hundred identical animals to extract a single, generalizable 'proof' or piece of 'truth'. Clinical practice deals with a hundred variables at one time within one animal from among a clientele of non-identical animals in order to optimize a mix of outcomes intended to satisfy that particular animal's current needs and desires. Interdependence among the variables. Most factors in the illness, the disease, the patient and the setting are interdependent, and cannot be sufficiently isolated to allow their separate study. Practice as a human transaction involving at least two people is too complex to be analysed one factor at a time when the interaction stimulates unpredictable responses. Ambiguous data. Words have many usages. People not only assign different interpretations to the same words, they assign different 'meanings', especially according to the threat or hope they may imply. The perceptual data gleaned from physical examination may be difficult to specify exactly or to confirm objectively. The accuracy and precision of investigational data and their reporting can be low, and are frequently unknown. Differing goals between science and practice. Science strives for exact points of propositional knowledge, verifiable by logical argument using objective data and repetition of the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)