Concepts can be classified in many ways (see the p.s to this post). In fact, things like a unicorn and God and rabbits horns can be considered as different types of pannati from trees. Trees, computers, humans, Robert, Howard, are the shadows of what is really there - and what is really there are only namas and rupas, mentality and matter, insignificant dhammas that can barely be said to exist because they pass away instantly. These concepts are more deluding than concepts like unicorns (which we know have no reality).
Because of accumulated avijja, ignorance, these type of concepts (pannatti) delude and instead of being given their correct status - as neccessary designations* - they are assumed to be actual. And that is where all problems begin and end.
*Note that these designations happen long, long before they are linguistic labels. What is called a thought in conventional language is comprised of billions of momentary arisings which repeatedly take a concept as object and may include metally naming it. Becuase of this repetition - and the lack of insight into the actual dhammas - the illusion of permanence is solidified.
The commentary to the UDANA ( translation by Peter Masefield from PTS) (p71,vol1, enlightenment chapter)
"it is ignorance since it causes beings to dart among becomings and so on within samsara.., it is ignorance since it darts among those things which do not actually exist (i.e. men, women) and since it does not dart among those things that do exist [.e. it cannot understand the khandas, paramattha dhammas).
Howard: the example you give of the thought of a flying purple elephant makes it clear to me that a so-called (individual) thought is actually a whole process, a sequence of cittas whose objects are various paramattha mind-objects (such as mental images). Is it always so that a thought is such a sequence?
Yes, that is right. What we call a thought in conventional language is a long and complex series of different processes. This is explained in detail in the commentaries.
Howard: Here you say "If ... you think of them, again the object is concept, not real; but the thinking process is real." Now, what if Christine does not think of them, but "merely looks" at them. That so-called mere looking at them is really a lot more than just looking. There is a big mental process transpiring that recognizes (perhaps wordlessly) two "entities" who are "people", and they are "people who she knows",
Yes, this is right. As I said above the conceptualising happens long before any naming has time to occur. Even babies and animals who have no linguistic abilities are fully involved in processes of conceptualising. However, animals and babies cannot yet expand concepts into the religions, sciences, and the craziness and wonder of civilisation. I think it can only be known by direct insight whether this is true or not and that is why the Buddha's teaching is ehipassiko - come and see. Which is why I believe vipassana is not a matter of doing something to get something ; instead it is simply the developing of insight into what is real and what is not. All these processes, the realities and the concepts are happening every moment of the day. They do not have to be searched for - they only need to be seen.
p.s. Abhidhammattha Sangaha Ch VIII, section 4, on pannattis:
i) formal concept (santhana pannatti) corresponding to the form of things, such as land, mountain or tree, which are so designated on account of the mode of transition of the elements.
ii) collective concept (samuha pannatti), corresponding to modes of construction of materials, to a collection of things, such as a vehicle or a chariot.
iii) conventional concept (sammutti pannatti), such as person or individual, which is derived from the five khandhas.
iv) local concept (disa pannatti), a notion or idea derived from the revolving of the moon, such as the directions of East or West..v) concept of time (kala pannatti), such as morning, evening
vi) concept of season (masa pannatti), notions corresponding to seasons and months. The months are designated by names, such as Vesakha.
vii) concept of space (akasa), such as a well or a cave. It is derived from space which is not contacted by the four Great Elements.
viii) nimitta pannatti, the mental image which is acquired through the development of samatha, such as the nimitta of a kasina
See http://www.abhidhamma.org/sujin3.htm Realities and Concepts Sujin