what does "think in a perpendicular position"mean?
Is there the faintest shadow of a reason why you should not be able to think as well in a perpendicular position before an audience as you can sitting down? Is there any reason you should pay host to butterflies in your stomach and become a victim of the “trembles” when you get up and address an audience? Surely, you realize that this condition can be remedied, that training and practice will wear away your audience fright and give you self self-confidence.

Question

what does "think in a perpendicular position"mean?
Is there the faintest shadow of a reason why you should not be able to think as well in a perpendicular position before an audience as you can sitting down? Is there any reason you should pay host to butterflies in your stomach and become a victim of the “trembles” when you get up and address an audience? Surely, you realize that this condition can be remedied, that training  and  practice  will  wear  away  your audience fright and give you self self-confidence.

Answer

I believe  a "perpendicular position" refers to standing up because you're perpendicular to the floor. The author is asking why people become more anxious about standing and speaking in front of people than they do if they remain seated. You should be able to think as well in a standing position as in a seated position.

Не упустите возможность выучить язык, не выходя из дома. Ознакомьтесь с подборкой опытных преподавателей и запишитесь на свой первый урок прямо сейчас!