Hi,
I'm writing a document called "Ars Magica for Dummies" for a couple of new players in our group, and I suddenly became unsure about a rules issue myself. A Magus casts a Touch spell on himself, but fails with, say, 5. Can he still affect himself with this Touch Spell without any penetration? I'm just not sure myself.. 
Eirik
You need to penetrate your own Parma to cast Touch spells on yourself, unless you lower your Parma momentarilly.
But can the Magus suppress his Magic Resistance due to Forms this way? He has at least 0 i Magic Resistance against all Forms, right?
E.
That by the way is easier said than done. The Concentration check required to suppress your Parma is equivalent to maintaining another spell, which makes it pretty darned hard to cast a spell while doing it.
The question of Penetration on one's self is often hand waved by many SG's. The rules are quite demanding, absurdly so to some sensibilities. Altho' it's not the RAW, many troupes simply state "Parma was designed as a defensive mechanism, and as such has no effect on your own spells against yourself" - end of problem.
He can suppress the whole thing (p. 85)
That's ease factor 12 (p.82). if you've got concentration 1 with an appropriate specialization and a stamina of 3 you still need to roll a 7 (or roll a 4 and burn a point of confidence) Those are typical numbers for a new magus in most of the sagas I've done since fifth edition came out. It's not an impossible task, but if you don't have something going for you beyond typical starting numbers you'll fail more often than you'll succeed.
Reach up a pluck a few hairs prior to casting. Most magi are also know their own name and nativity horoscope so they can usually count on a x6 penetration multiplier. Merciful SGs (of which there are a few) might also allow the magus involved to use "Blood Relative" or "Symbolic Representation" for an even greater bonus...
Those three give you a x7 multiplier (they provide bonuses to the multiplier, the multiplier starts at 1)
If your SG is not overflowing with mercy, you could certainly carry around a representation of yourself and, if you're industrious, a daily horoscope (like you'd have to carry a nativity horoscope) that puts it up to *9.
Just don't let any of those things out of your control...
Alternatively you could do a MuVi to alter the Range of the spell you want to cast on yourself to Personal. The Concentration roll there is easier (Int + Conc. vs. 9 -- p. 159) than it is for suppressing your own Magic Resistance, and that way you don't have to lower your defenses at all.
But a R:Personal spell cast on yourself (naturally) isn't rested, so no need to suppres Parma Magica.
And IMHO a Parma which is suppresses offers no restsiance at all, not even from Forms, on spells cast on you. IMS we routinely supres our Parma for another PC magus to cast a spell on us if we can't cast it ourselves. They even let my Tremere - Vries van Breinermoor - cast teleportation spells on them...
But we enforce the rule that anything but R: Per cast on yourself must Penetrate, or you must supres Parma. We to tend to forget Concentration rolls though. Good point, we need to enforce that!
I believe that was his point.
This is straight from the rulebook, so yeah.