What is meant by "straightforward facts" in the paragraph below?

"You could see from afar the white of their eye glistening. They shouted, sang; their bodies streamed with perspiration; they had faces like grotesque masks—these chaps; but they had bone, muscle, a wild vitality, an intense energy of movement, that was as atural and true as the surf along their coast. They wanted no excuse for being there. They were a great comfort to look at. For a time I would feel I belonged still to a world of straightforward facts; but the feeling would not last long. Something would turn up to scare it away." (Heart Of Darkness).