piotr wrote:Why in Pāli it is said "rūpaṃ anattā, [...] viññāṇaṃ anattā". And not "rūpaṃ anattaṃ, [...] viññāṇaṃ anattaṃ"?
In this passage 'anattā' = 'na' + the substantive 'attā' + an assumed copula (atthi): "is not self".
When 'na' is made into the first element of a compound it becomes 'a-'. As the following component in the compound begins with a vowel, for the sake of euphony 'a' changes to 'an-'.
And so unlike 'anicca' and 'dukkha', 'anattā' in the above passage is not modified to agree with the neuter 'viññāṇaṃ' because structurally 'anattā' is not an adjective. Rather it is a noun employed predicatively. This is how anattā is most commonly used in the Suttas.
On the other hand, there are a few instances where anatta is used in an attributive rather than predicative fashion, and here it will be inflected as an adjective. For example in the Udāna Sutta (SN. iii. 56) we meet with the following:
anattaṃ rūpaṃ 'anattā rūpan' ti yathābhūtaṃ nappajānāti, anattaṃ vedanaṃ 'anattā vedanā' ti yathābhūtaṃ nappajānāti, anattaṃ saññaṃ 'anattā saññā' ti yathābhūtaṃ nappajānāti, anatte saṅkhāre 'anattā saṅkhārā' ti yathābhūtaṃ nappajānāti, anattaṃ viññāṇaṃ 'anattā viññāṇan' ti yathābhūtaṃ nappajānāti.
"He does not understand selfless [or insubstantial] matter as it really is, as 'matter that is not self'. He does not understand selfless feeling... etc.