Recently I read a post by a US Classical Tai-chi sifu in his blog with this heading '"Right" Mental states do not overcome bad postures'. I certainly agree with this statement. A statement put in such a way that it won't fail to be saying a "general truth" or "general maxim". The flip side of this general maxim is '"Right" posture cannot drive away, say, one's (bad) erotic thought'. You know what I mean....
A more interesting question is "Do we start our internal martial art practice (tai-chi included) as being with movement or without movement?" I understand that certain approaches in tai-chi (classical or otherwise) advocate starting movement in lesson one (no matter it is called "silk-reeling" or whatever). There are still many of these teachers around. But as I can see it, the current trend of coaching the internal martial arts in the Far East (including HK and the Mainland) is to start with the stationery form Zhan Zhuang. This trend has been influenced most by one Great Martial Art teacher: Grandmaster of Yi-style: Wang Xiangzhai (王薌齋).
In the practice of Zhan Zhuang, the correct "mental state" is very important (please refer to my post Zhan Zhuang 101 for details). Whereas Taoist meditative approaches generally focus on internal feeling of chi-movement, Master Wang focused on external visualization (like holding a big rubber ball with some weight). All in all, a mental state of being in the (meditative) zone will be essential.
It was said that Master Wang forbade his students doing ANY movement before he was satisfied with their individual progress in doing Zhan Zhuang. Needless to say, teachers or coaches nowadays will generally be more "lenient", lest their students might doubt they have nothing "useful" to teach except "just stand there (and mind your mental state)", afterall most are not as famous as the late Master Wang!