Relations, like properties, are constituents of propositions. The ontology should just follow whatever you think of propositions. Do you agree? — Tate
For a Direct Realist and Idealist, the ontology of the external world would follow from their propositions. For the Indirect Realist, it wouldn't.
What in the world are objects, properties and relations
In my mind, I could have thoughts about things and thoughts about propositions. For example, I could name the object comprising my pen and the Eiffel Tower a "peffel". The "peffel" has the property of being extended in space, with the pen at one end and the Eiffel Tower at the other. The proposition "the peffel is north of Lyon" describes a relation.
The question is, is there an external world, and if there is, do these objects exist in it. Assuming there is an external world, an important question would be, do relations exist within it. Because, if relations don't exist in the external world, then neither do properties, and neither do objects such as peffels, Eiffel Towers, trees, apples and tables.
Wittgenstein
1 The world is everything that is the case.
1.1 The world is the totality of facts, not of things
2.01 An atomic fact is a combination of objects (entities, things)
4.123 A property is internal if it is unthinkable that the object does not possess it
As regards Wittgenstein's
Tractatus, the word "world" can be read as being either inside or outside the mind of the observer. As Wittgenstein states that the world is everything that is the case as a fact, as knowledge, and not a justified belief, and as our only knowledge is within the mind, I read Wittgenstein's world as also being in the mind of the observer.
The
Tractatus therefore does not address the question of whether relations exist outside the mind. For Wittgenstein, "the peffel is north of Lyon" is a proposition in the mind with the same logical form as the fact in a world existing in the mind that the peffel is north of Lyon.
Bertrand Russell
As regards Bertrand Russell, I read Russell as a believer in Realism, a believer in the existence of a world outside the observer's mind, in the existence of a mind-independent world and where relations do exist. Therefore, his thoughts on relations in the world are relevant to the current topic. For Russell, atomic facts exist in a world independent of minds, and where mathematical and logical truths such as 2 + 2 = 4 - must be unconditionally true. For Russell "the peffel is north of Lyon" is an atomic proposition in the mind that corresponds with the atomic fact in the world that the peffel is north of Lyon.
Internal and external relations
As regards an internal relation, a given property, such as being a house, entails another property, such as being a place for living. As regards an external relation, the property being a house is external to the property being a mode of transport
FH Bradley
Famously, Bradley brought a vicious regress argument against external relations. In his original version of 1893, Bradley presented a dilemma to show that external relations are unintelligible: either a relation R is nothing to the things a and b it relates, in which case it cannot relate them. Or, it is something to them, in which case R must be related to them.
Bertrand Russell's counter-argument
However, in the journal
Mind 1910-1911, Bertrand Russell argued against Bradley's Regress Argument, rejecting internal relations in favour of external relations. He argued that what distinguishes a complex from a mere aggregate is that relation in a unified complex relates whereas a relation in an aggregate does not relate, and is just a member of the aggregate.
Bradley's response
Bradley found Russell's reply unsatisfactory, asking Russell to elaborate further of the difference between an aggregate of entities and a unity of those entities. However, Russell did not feel that there was anything more to be said in that the difference between relating and non-relating relations is a primitive which cannot be further explained.
Bertrand Russell's The Problems of Philosophy 1912
Russell wrote: "Consider such a proposition as "Edinburgh is north London." Here we have a relation between two places, and it seems plain that the relation subsists independently of our knowledge of it. When we come to know that Edinburgh is north of London, we come to know something which has to do only with Edinburgh and London: we do not cause the truth of the proposition by coming to know it, on the contrary we merely apprehend a fact which was there before we knew it. This conclusion, however, is met by the difficulty that the relation "north of" does not seem to exist in the same sense in which Edinburgh and London exist. If we ask "Where and when does this relation exist?" the answer must be "Nowhere and nowhen." There is no place or time where we can find the relation "north of." It does not exist in Edinburgh any more than in London, for it relates the two and is neutral as between them. Nor can we say that it exists at any particular time. Now everything that can be apprehended by the senses or by introspection exists at some particular time. Hence the relation "north of" is radically different from such things. It is neither in space nor in time, neither material nor mental; yet it is something."
The problem with Russell's explanation
Russell wrote that the relation subsists rather than exists, is neither in space nor in time, neither material nor mental; yet it is something.
Subsist means to have timeless or abstract existence, as a number, relation, etc. To say that relations exist outside of time and space is no more an explanation that saying that they have magical powers.
Does Bradley's argument fail by logic
@Banno pointed out the belief that Bradley's argument fails because he used Aristotlean rather then Fregean logic. However, Bradley's debate with Russell in 1910 to 1911 was more than 30 years after Frege's breakthrough
“Begriffsschrift, eine der arithmetischen nachgebildete Formelsprache des reinen Denkens” of 1879, which Bradley must have been aware of as it marked a turning point in the history of logic, using the ideas of functions and variables.
Aristotle's subject-predicate was limited by the propositions all S is P, all S is not P, some S is P and some S is not P, whilst Frege borrowed from Boole and de Morgan the idea that propositions can be considered as variables that can have the values true or false. It could be argued that Bradley's regress argument - either a relation R is nothing to the things a and b it relates, in which case it cannot relate them. Or, it is something to them, in which case R must be related to them - is more Fregian than Aristotlean.
Relations problem with limiting the number of possible objects
If relations exist, the "peffel" is a complex object being a composite of the simple objects "pen" and "Eiffel Tower". These simple objects become complex objects when their individual parts are related. But the pen is also legitimately in relation with the Empire States Building, another complex object that can be named. In fact, as the pen is in relation with every other object existing in the Universe, each of these complex objects may also be named. But in addition, each atom in the pen is also in relation with every other atom in the universe, each of these complex objects may also be named.
For example, given four objects A, B, C and D, there are 14 possible complex objects, each of which can be named, for example the complex object ABD. If relations exist, then starting with 4 real things, we end up with 14 real things.
The question for relations is, if relations exist, where did these 10 new things come from.
Relations problem with information
Given the pen, for example, where exactly is the information that the pen is in a relation with the Eiffel Tower, or the Empire States Building, or even a particular rock in the Andromeda Galaxy. Can any investigation of the pen ever reveal this information. Can any investigation of the space between the pen and a particular rock in the Andromeda Galaxy ever reveal that there is a pen at one end an a rock at the other. Can an investigation of the rock in the Andromeda Galaxy ever reveal this information.
The question for relations is where exactly is the relation, if it doesn't exist in either each thing it relates or the space between the things it relates.
Relations problem with time
Are relations instantaneous, or is time required for the establishment of a relation between two objects spatially separated, for example, the pen and the rock in the Andromeda Galaxy. If the relation is instantaneous, how does this fit in with our scientific knowledge to date that noting can travel faster than the speed of light. If the establishment of a relation between two objects is limited by the speed of light, by what mechanism does the information travel between the two objects.
Are there "relatons" still to be discovered by the Large Hadron Collider ?
Conclusion
There are practical issues if relations do exist outside the mind of an observer, and until answered, the belief that relations do exist cannot be fully justified.