Ceci n'est pas une pipe

Individual viruses just like individual  people or any other concrete objects cannot be “defined”; they can only be named in a manner  reminiscent of baptism or they can be pointed at ostensively.  … Since species are actually fuzzy sets, i.e. polythetic classes, creating and delineating a virus species is more a matter of opinion and convenience rather than of logical necessity.

–Van Regenmortel MH (2003). Viruses are real, virus species are man-made, taxonomic constructions. Arch Virol 148:2481–2488. DOI:10.1007/s00705-003-0246-y