(It is conceivable that there may have been an intruder that the dog did not detect, but that does not invalidate the argument; the first premise is "if the watch-dog detects an intruder."
(Bisa dibayangkan bahwa mungkin ada penyusup yang tidak terdeteksi anjing, tapi itu tidak membatalkan argumen; premis pertama adalah "jika anjing mendeteksi penyusup."