Before answering the question, keep in mind that I am a second-year Biology student, with a lack of formal study in Philosophy.
Original post: I believe, all definitions, words, and concepts (abstract ideas) are arbitrary (here I define arbitrary as anything resulting from personal choice). It seems logical to me, that the distinctions humans and all other living organisms make between any two things (by using the word 'things', I am implying distinction exists outside of consciousness), must be a product of personal choice.
In other words, we (i.e., humans and other living organisms. Side note: this is itself a distinction that does not necessarily exist outside of our consciousness. I for one, personally, do not see a reasonable distinction between living organisms) make distinctions between 'things', which we arbitrarily define (perhaps for an evolutionarily advantageous reason), but the 'universe' excluding living organisms, does not.
However, under my 'logical' reasoning, if the distinction between 'life' and 'non-life' is also arbitrary, (we can be seen as a part of the universe, or indifferent from the universe; either way, our consciousness is a part of the universe, or is the universe, and thus, our concepts, definitions, and words do exist in the universe, as do the concepts, definitions, and words of other individuals and living organisms) then all of these possible arbitrary distinctions exist in the universe. This also holds that abstract ideas are not existing separately from the physical world.
Edit 1: for those arguing intuition, 'intuition' changes person-by-person and most likely, species-by-species.
Edit 2: to clarify, I am arguing that the distinctions that we make between matter (the substance of everything), which can be represented as lines we draw between stuff, are arbitrary, not just the names we have for those distinctions. This is what I believe to be a result of humans not knowing nature (i.e., everything, the universe). Since we do not know if matter is infinitely divisible, finitely divisible or even divisible in the first place (very unlikely, in my opinion), we cannot distinguish between any two things, non-arbitrarily. For example, at what point in organisation, from molecules to cells, does 'life' suddenly appear? Here is another question, if we are constantly replacing the matter, that composes 'us', how do we define or distinguish an individual organism, at the atomic scale? Again, I suspect we cannot answers these questions in a definite, non-arbitrary way, because we do not know our reality.
One Philosophy I have is: to know anything is to know everything. Which I believe is relevant here.
Edit 3: after reading the excellent response by Michael, here is what I think I understand. Firstly, in science, we can never prove anything, as we cannot test every possible hypothesis. Secondly, if we do not understand consciousness, how can we understand anything? Nevertheless, patterns exist in nature (which I interpret as nature being repetitive in some respects), living organisms form concepts (e.g., distinctions) by identifying patterns (according to Michael). Since living organisms are exposed to same set of patterns (i.e., the same reality), we form similar concepts by inevitably identifying the same patterns, every now and then. Therefore, concepts constructed by living organisms (attempting to understand their world) reflect the nature of nature. This refutes the claim that all concepts are subjective (before I used the word arbitrary, a word which I was misusing, thank you NotThatGuy for the correction). However, an interesting point to add is this, living organisms identify natural patterns via their biological senses. Because sensory capability varies from individual to individual and from species to species (due to anatomical and physiological differences between living organisms), one living organism may be able to identify a pattern in nature that another living organism is unable to identify (due to differences in their sensory capabilities). If this hypothesis is valid, one would expect individuals with similar sensory capabilities to identify a similar subset of patterns in nature (i.e., what Michael describes as 'a similar set of lower [in terms of abstraction], more concrete concepts'), whereas individuals with greater differences in their sensory capabilities (e.g., a deaf person versus a person with normal hearing, or a dog versus an echidna) would be expected to have a more dissimilar set of ‘lower, concrete concepts'. In other words, it could be expected that individual organisms have an inherent bias in the way they sense their world. Furthermore, one would expect individuals with similar sets of 'lower, concrete concepts' to have a more similar understanding of nature and thus, language and communication between these individuals would be easier, compared to communication between organisms with different sets of concrete concepts.
The question of how we measure differences in sensory capability between individuals, of the same and of different species, thoroughly, is another problem altogether. Another interesting point to add is that differences in how living organisms process sensory information (i.e., what sensory information does a specific organism decide to accept and reject, does this differ compared to another individual organism) would play a role in the patterns they are able to identify in nature (think of the senses as the hardware and the brain or processor as the software).