Why is contracting for human milk permissible in hiring?

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The permissibility of contracting for human milk, despite it being a physical substance (*ayn*), is granted as a concession (*rukhsah*) because nothing else can substitute for it, and necessity demands its consumption for the preservation of the human being. This concession is specific to human beings and not granted for other animals due to the necessity of preserving human life and the need to sustain it.