[220] Travel also through the greatest and most perfect being, namely this world, and consider all its parts, how they are separated in respect of place and united in respect of power; and also what is this invisible chain of harmony and unity, which connects all those parts; and if while considering these matters, thou canst not easily comprehend what thou seekest to know, persevere and be not wearied; for these matters are not attainable without a struggle, but they are only found out with difficulty and by means of much and great labour;