Sforno on Genesis 25:31/ספורנו על בראשית כ״ה:ל״א

1מכרה כיום, seeing that as of now all your interests are focused on your work so that you are so tired that you cannot even recognise a dish of lentils for what it is, there is no question that you would not have the time or energy to perform the duties involved in the obligations associated with being a firstborn