In the late Middle Ages the use of force to defend legal rights, property, and even nation, was considered to be wholly legitimate. On an individual basis, for those who experienced combat on the battlefield, or for those civilians who were unfortunate enough to be faced with the threat of invasion, defending one's life was considered a self-evident necessity. Accordingly, the employment of violence to defend oneself was widely assumed to be entirely legal. Indeed, the precept of self-preservation was perhaps the most fundamental of all the notions of natural law, as Cicero observed:

The Christian abhorrence of suicide lent even more force to this command for self-preservation. Concomitant with this natural instinct and Christian duty was the notion of a right to self-defence. At the beginning of the late medieval period, repelling force with force was seen as a universal human right. Innocent IV stated that, ‘It is lawful for every man to move war in defence of himself and his goods’; Raymond of Peñaforte asserted that, ‘it is always lawful to meet force with force’; and Aquinas concluded that self-defence was licit because, ‘according to the laws, “it is legitimate to answer force with force provided it goes no further than due defence requires”’. All three scholars were building on assumptions presented in the Decretum, where Gratian had marshalled sources to prove that individuals had a right to defend themselves. An Isidorian text in the Decretum explained that:

The text cited by Gratian describes a number of fundamental natural precepts but, as we can see, the right of self-defence by repulsing ‘force with force’ was considered as natural as procreation or personal liberty. It is this aspect of natural law that I will be examining – the natural precept of self-preservation, and the natural right of self-defence using force.