Teachings about the Way Things Are
The truth claims of humanism are largely to do with our understanding of the world and ethics (how we relate to and treat each other and the natural world, and why). The source and evidence for both sets of truth claims lie in human experience and knowledge gained by the scientific method rather than in authority or tradition.
Humanists understand the world through observation and through reasoning and hypothesizing (not necessarily in that order), processes normally associated with science and not uniquely humanist, of course, though exclusive reliance on them does differentiate humanists from some religious believers.
The distinctive truth claims made by humanists are ethical: that our relationships and moral values are founded on human nature and experience alone and concern for others does not require an external source or authority.