The first is intelligibility and ease for the majority. This principle, in turn, leads to two others: that of maximum lexical internationality and that of grammatical ease.
The second principle is the adoption of the most spoken languages of the planet. We have taken, as reference, the major European tongues (English, Spanish, Portuguese, French, German, Italian), along with Chinese, Russian, Hindi, Arabic, and Persian. LdP also includes a certain number of words from other languages. This principle is important for the neutrality as well as psychological acceptability of an auxiliary language. Only a language that is psychologically acceptable for the majority can provide a friendly psychological field for international communication and hence for unification.
The fact that demonstrates the importance of psychological acceptability is that people of a subjugated country never accept the invaders' language as their own, even if they understand it well. In no way is intelligibility the only requirement for an interlanguage. We are convinced that elements of one's native tongue in a constructed language evoke strong positive emotions and considerably increase one's interest and motivation.
The third principle is that of naturalism. LdP is based on the living languages and tends to incorporate words without serious deformation. We do not invent non-existing words, and we do not revive the words and grammar elements of dead languages or reconstruct lost common roots. We proceed from the current linguistic situation and take words as well as grammar elements from reality.