أَوْ بِوَاسِطَةِ عُرْفٍ عَامٍّ
Or through a generally shared convention.
Some resemblances are not equally obvious to all people by direct observation, but are familiar within ordinary linguistic or cultural usage. Such a metaphor can be understood through shared convention, without requiring an elaborate new argument.
Common usage is not merely a dictionary entry; it includes habitual associations between an image and a quality. Those established associations enable listeners to recognize the transferred meaning quickly.