A thorough analysis of the sensitivities of the four Stokes profiles to the physical quantities involved in the local thermodynamic equilibrium (LTE) line formation is presented. We point out the ambiguities on the definition of a mean depth of formation of a given spectral line and on the use of Contribution Functions for the ascription of a measure to a given depth in the atmosphere. Response Functions behave like partial derivatives of the Stokes spectrum at a given depth of the atmosphere. They provide the sensitivities of the observed spectrum to the physical quantities characterizing the state of the atmosphere. After a theoretical generalization of any measured parameter over the spectrum, we extend the concept of Response Functions for such parameters; in detail are discussed the properties and sensitivities of the equivalent width, the Stokes V peaks distance, the line-ratio method, and the centre of gravity method. Of particualr interest are the following results : (1) a constant and longitudinal magnetic field can desaturate a spectral line with a Zeeman pattern other than a pure Zeeman triplet; (2) saturation is readily understood as a consequence of an enhencement of the photon supply; and (3) methods to measure magnetic field strength are sensitive to temperature variations, mostly if a field strength gradient is present through the photosphere.