Sentence:  Much less can we afford to follow them into the fantastic combination in which they have been sometimes represented as constituting a sort of barbarian <WSD>headspring</WSD> to the liberty of civilized men ; to whom politic society is thought to be a means of preserving, in part only, a freedom enjoyed by savages in much larger measure, nay in perfect plenitude. 
Meaning of the word marked with <WSD> tags:  A basis or foundation.