Friendship. Seneca, Epistulae 1.3.1-2

A true friend is one you trust the way you trust yourself. You can listen to me reading this passage <here>.

Epistulas ad me perferendas tradidisti, ut scribis, amico tuo; deinde admones me ne omnia cum eo ad te pertinentia communicem, quia non soleas ne ipse quidem id facere: ita eadem epistula illum et dixisti amicum et negasti. Itaque si proprio illo verbo quasi publico usus es et sic illum amicum vocasti quomodo omnes candidatos 'bonos viros' dicimus, quomodo obvios, si nomen non succurrit, 'dominos' salutamus, hac abierit. Sed si aliquem amicum existimas cui non tantundem credis quantum tibi, vehementer erras et non satis nosti vim verae amicitiae. Tu vero omnia cum amico delibera, sed de ipso prius: post amicitiam credendum est, ante amicitiam iudicandum. Isti vero praepostero officia permiscent qui, contra praecepta Theophrasti, cum amaverunt iudicant, et non amant cum iudicaverunt. Diu cogita an tibi in amicitiam aliquis recipiendus sit. Cum placuerit fieri, toto illum pectore admitte; tam audaciter cum illo loquere quam tecum.

The letters you sent me were to be delivered, you write, to your friend. Then you warn me to avoid telling him all your business, since you are not accustomed to do the same. Thus, in the same letter, you call him friend and deny friendship. If you are using that word in some public sense, calling him friend the way we call all candidates for office gentlemen or address unknown strangers as sir, then here the matter ends. But if you esteem someone a friend whom you do not trust as much as you trust yourself, then you make a great mistake and show ignorance of the power of true friendship. Take the measure of all things with a friend, but first you must measure him. After friendship is granted, trust must follow; before it, there is judgment. They err egregiously here who overturn the order of operations taught by Theophrastus, causing their judgment to follow their affections rather than the other way round. Take your time when deciding whether to recognize another as friend. When you are ready, receive him with all your heart. Speak with him then as boldly as with yourself.