Russell (1961: 640):
H[um]e repeats many times the contention that what appears to us as necessary connection among objects is really only connection among ideas of those objects: the mind is determined by custom, and ' 'tis this impression, or determination, which affords me the idea of necessity'. The repetition of instances, which leads us to the belief that A causes B, gives nothing new in the object, but in the mind leads to an association of ideas; thus 'necessity' is something that exists in the mind, not in objects'.
In the view of Systemic Functional Linguistic Theory, both objects and ideas of objects are construals of experience as meaning. That is, they are construals within the semantics of language. How they differ is in terms of order of experience: objects are first-order construals (phenomena), whereas ideas of objects are second-order construals (metaphenomena). Causation or necessity can be construed at either order of experience, but in each case it "exists" as a construal of experience as meaning; they are the mental projections of sensers engaged in mental processes (or the verbal projections of sayers engaged in verbal processes).