It means a natural necessity proceeding from physical causes alone without the intervention of man. It is an act, event, happening, or occurrence, due to natural and inevitable accident, or disaster; a natural and inevitable necessity which implies entire exclusion of all human agency which operates without interference or aid from man and which results from natural causes and is in no sense attributable to human agency. It is an accident which could not have been occasioned by human agency but proceeded from physical causes alone.