Two third tones rule in Mandarin.

An interesting thing happened to me yesterday. For the first time since I started learning Mandarin - and that would include 3 years of demanding studies at our university's Chinese department, 5 years of complete immersion in Taiwan and 10 years of working as an interpreter for the police, judiciary, prosecution.. all in all a daily grind of 17 years..  I managed to 'feel' and not 'know' that a 3-3 tonal combination in Mandarin should be a 2-3 combination. 

In other words, I 'felt' and not 'knew because I had learned the rule', that when there are two third tones in close proximity in Mandarin, that it "can't be like that" and that the first of the two tones must be pronounced as the second tone. (The full rule is a bit more complex, but for the sake of simplicity, this is what I want to go with for now). 

Before that, all that time, the whole 17 years, since I started studying Mandarin, when I spoke Mandarin, I had to pay attention to what tones would follow in my speech, and all that time when a 3-3 combination was approaching, I had to consciously 'initialize' the 3-3 > 2-3 rule and pronounce the combination as 2-3. 

Even after 17 years it was still a bit difficult and a bit tiring in the sense that it cost me my attention, and it put slight pressure on the syntax and the overall sequence of thoughts around the 3-3 point in my speech. 

Yesterday, as a form of practice, I was reading out loud a transcript of a podcast with a native-speaker friend. We've been reading it regularly for about 2 weeks now and suddenly I came to a point in the text where "Alice很努力" appeared and because I was reading too quickly I went on to read it as "Alice hěn nǔlì " and I said... "Wait a second, that can't be." 

The thought came to me automatically and it wasn't because I had remembered the 3-3 > 2-3 rule or that I had felt like a 3-3 combination was too difficult to pronounce (which I heard as an explanation as to why a 3-3 combination has to be changed to 2-3 one). 

The thought that a 3-3 combination 'just can't be' came to me automatically, and the reason was that, as I am now practicing supervised loud Mandarin reading the most I ever had, I managed to internalize/automate all the possible tonal combinations in Mandarin as very solid individual entities (1-1, 2-3, 4-1 etc.), and 3-3 just isn't one of them.

So, the reason, why a 3-3 tonal combination has to be changed into a 2-3 one is not because there is a rule that says so, or because it would be too difficult to pronounce a 3-3 combination, but simply because there is no 3-3 combination in Mandarin in close-flow proximity. It just doesn't exist. Under certain circumstances, where syntax allows, there can be two syllables with a 3rd tone following each other separated by a slight pause but a solid 3-3 combination in Mandarin simply doesn't exist. 

This might sound like a minute detail, but it really isn't. I think all of us remember the mental gymnastics we had to do when starting out with Mandarin having to be on the lookout for a 3-3 combination, then initialize the 3-3 > 2-3 rule a actually remember what we were trying to say and continue our speech. As my example shows, this does not necessarily improve over time. 

Just without giving it more thought, maybe beginners shouldn't be told:

when two 3rd tones appear in close proximity (meaning no pause, even a slight one), only the last 3rd tone should be pronounced a 3rd tone and all preceding 3rd tones should be pronounced as the 2nd tone.

but rather:

two 3rd tones cannot appear in close proximity in Mandarin because a 3-3 combination in Mandarin doesn't exist. It can be either 2-3 or 3 pause 3, but not 3-3. And all 3-3 combinations must be pronounced 2-3, or maybe that "since a 3-3 combination does not exist in Mandarin, you have to make sure a 3-3 combination does not appear in your speech, even if several 3-3-3-3-3.. syllables are next to each other and you do it by either raising all but the last 3rd tone into the 2nd tone, or make a pause, where syntax allows.