He is referring that it is natural for human beings to speak the truth, even if it causes harm.
Explanation:
Thoreau believes that human beings are destined to always speak the truth, as this is something that happens naturally in the formation of human beings. In this case, Thoreau believes that all individuals will speak the truth regardless of what it may cause, that is, even if the truth causes pain and suffering, or causes negative things, it will be spoken and cannot be avoided.
