Malthus is responding to William Godwin's Political Justice, in which Godwin argued that the institution of property causes inequality and then "proverbial" "wretchedness", as the poor are unable and unwilling to reduce their family size:
First then it is to be observed, that, in the most refined states of Europe, the inequality of property has arisen to an alarming height. Vast numbers of their inhabitants are deprived of almost every accommodation that can render life tolerable or secure. Their utmost industry scarcely suffices for their support. The women and children lean with an insupportable weight upon the efforts of the man, so that a large family has in the lower order of life become a proverbial expression for an uncommon degree of poverty and wretchedness. If sickness or some of those casualties which are perpetually incident to an active and laborious life, be superadded to these burthens, the distress is yet greater. William Godwin, An Enquiry Concerning Political Justice, , p. 34.
See "The Anti-Exchange Coalition: Godwin Remakes the World" by David M. Levy and Sandra J. Peart.