If you tax too high, the revenue will yield nothing.
Emerson uses taxation as an example of a broader principle: pushing a demand too far can defeat the purpose it was meant to serve. If taxes become excessively burdensome, he suggests, they may reduce the activity, compliance or economic capacity from which revenue is collected. The result can be that attempts to raise more money eventually produce less.
Within Compensation, the remark is not presented as a detailed theory of taxation. It serves Emerson’s wider argument that gains and pressures are often balanced by counteracting effects. In economic terms, the line points to the possibility that government policy can create behavioural responses that limit its own effectiveness. It therefore illustrates the importance of considering incentives, practical limits and unintended consequences when designing systems of taxation or other forms of public extraction.