From Diognetus...

I. 6. From Diognetus, not to busy myself about trifling things, and not to give credit to what was said by miracle-workers and jugglers about incantations and the driving away of daemons and such things; and not to breed quails for fighting, nor to give myself up passionately to such things; and to endure freedom of speech; and to have become intimate with philosophy; and to have been a hearer, first of Bacchius, then of Tandasis and Marcianus; and to have written dialogues in my youth; and to have desired a plank bed and skin, and whatever else of the kind belongs to the Grecian discipline.

Diognetus was a master of painting, but it is obvious his instructions to Marcus included much, much more. Anthony Birley speculates that Marcus received his first philosophical training, perhaps unofficially, from Diognetus. This seems certain, as Marcus explicitly states that he learned to become “intimate with philosophy” due to Diognetus.

…not to busy myself about trifling things…

This is a theme that will come up again and again in the Meditations. Since we will see much more of it later, I’ll refrain from discussing it here.

…and not to give credit to what was said by miracle-workers and jugglers about incantations and the driving away of daemons and such things;

The passage about exorcism of demons is almost universally believed to refer to the Christians. Not only Christians, however, but many people working as magicians or miracle-workers were striving for attention in this time period. More important than the historical background of this passage, however, is the implicit reliance upon Reason. We will see much more of this topic.

…and not to breed quails for fighting, nor to give myself up passionately to such things;

Again, this is a trifle that should not demand undue attention. Also of interest is that Fronto had a great passion for quails. Fronto at one time had a great influence over Marcus.

…and to endure freedom of speech;

As an almost absolute monarch with very few checks or balances, it must have been difficult to endure freedom of speech. This does not mean only what moderns think of it, such as freedom to speak against the government, but also personally. Marcus struggled with people who he believed wasted his time, speaking of inconsequential or negative things. He was learning to control his reaction to this speech, since he couldn’t control the speaker.

A great example of this is illustrated in I.7 regarding his old teacher Herodes. See that post for more information.

…and to have been a hearer, first of Bacchius, then of Tandasis and Marcianus;

Unfortunately I know nothing about these three lecturers. The assumption is that they lectured upon philosophy, perhaps Cynicism, based upon what Marcus says later about the "Grecian discipline."

…and to have written dialogues in my youth;

This exercise has been used for thousands of years. It remained one of the best ways for a student to demonstrate his knowledge of tenets and possible arguments against them.

…and to have desired a plank bed and skin, and whatever else of the kind belongs to the Grecian discipline.

About the time Marcus first began to be taught by Diogenetus, he became obsessed with living the philosophic lifestyle. The Historia Augusta states: “He studies philosophy with ardour, even as a youth. For when he was twelve years old he adopted the dress and, a little later, the hardiness of a philosopher, pursuing his studies clad in a rough Greek cloak and sleeping on the ground; at his mother’s solicitation, however, he reluctantly consented to sleep on a couch strewn with skins.”

Marcus was not a child playing at a philosopher; he was deadly serious. It may seem absurd for such a privileged youngster to deprive himself so, but Marcus was making a major attempt to better himself by emulating people and manners he greatly admired.

Seneca suggested the same formula. He said: “Set aside a certain number of days, during which you shall be content with the scantiest and cheapest fare, with coarse and rough dress, saying to yourself the while: ‘Is this the condition that I feared?’… If you would not have a man flinch when the crisis comes, train him before it comes. Such is the course which those men I have followed who, in their imitation of poverty, have every month come almost to want, that they might never recoil from what they had so often rehearsed. You need not suppose that I mean meals like Timon’s, or ‘paupers' huts,’ or any other device which luxurious millionaires use to beguile the tedium of their lives. Let the pallet be a real one, and the cloak coarse; let the bread be hard and grimy. Endure all this for three or four days at a time, sometimes for more, so that it may be a test of yourself instead of a mere hobby… There is no reason, however, why you should think that you are doing anything great; for you will merely be doing what many thousands of slaves and many thousands of poor men are doing every day.”

This is an extremely powerful formula. It must have been fairly common among the philosophically minded of the era. I have practiced it myself, although at times it seems embarrassing to explain what you are doing to others.