Observations that come from self-defining instruments are often unreliable. Such observations are hard to reproduce because they may vary even with respect to the same stimuli. Thus they are not of much use in exact sciences like physics which require instruments which do not define themselves. It is thus often necessary to use various engineered instruments like: spectrometers, oscilloscopes, cameras, telescopes, interferometers, tape recorders, thermometers etc. and tools like clocks, scale that help in improving the , quality and utility of the information obtained from an observation. 