This paper concerns the lower of a range of thresholds that control the response of condensed matter under loading in compression, from the ambient laboratory state to the point at which the bond strength is overcome and warm dense matter is formed. One oft-used term is yield stress and its variation with the rise time of the loading pulse are considered in this first paper. This threshold shows a correlation between the length scale swept by the rise of the pulse and the defect distribution within the target for a range of materials. Strain rate is also a useful term that reflects the evolution of the stress state within a target but must be defined for a particular volume element containing a particular defect distribution to reflect continuum conditions acting within and thus applies to a defined length scale within a target. This overview of behavior suggests concepts borrowed from rate-independent plasticity have served the community well but that to advance it may be necessary to use viscoplastic concepts in constitutive descriptions for the future.