How does the approach of someone who has adopted the precautionary principle differ from someone with a blind faith in substitutability, when it comes to a non-renewable resource like fossil fuels?

Someone with a blind faith in substitutability will think that the depletion of a non-renewable resource like fossil fuels is not such a serious problem, as they have faith that in the future other resources can cheaply be substituted for it.  Whereas someone who has adopted the precautionary principle will think that we should err on the cautions side and not simply suppose that other resources can be cheaply substituted for the non-renewable resource