Subconscious vs subconsciousness
Subconscious The OED has two adjectival senses and a noun sense. Examples of the noun date from 1877 (adjectival goes back a little earlier - early 19th C), and I suspect closely correlates with the emergence of psychology as a modern discipline. The adjectival examples have some religious connotations among them.
Subconsciousness has only noun senses.
What does seem to me to emerge from looking at this is that the noun subconscious tends to be used either with the definite article, or in describing that of a specific person e.g. his subconscious, Fred's subconscious etc, to refer to an aspect of the mind of an individual. Subsonsciousness seems to have a more amorphous use, an abstract noun which refers to the condition in general.
I do believe there to be a difference in the way the two words are used. Were I trying to psychoanalyse you (I do not have that ability, you will be relieved to know) I might refer to your subconscious. Were I talking about psychology in general I might say something like The current generation of cognitive psychologists place less emphasis on 'subconsciousness' as a route to understanding human behaviour.
Hmm. There seems to be some difference of opinion within the comments above. As far as I know, both subconscious and subconsciousness are nouns. Subconscious is also an adjective:
subconscious noun [S] uk /ˌsʌbˈkɒn.ʃəs/ us /-ˈkɑːn-/ › the part of your mind that notices and remembers information when you are not actively trying to do so, and influences your behaviour even though you do not realize it: The memory was buried deep within my subconscious.
subconscious adjective [before noun] uk /ˌsʌbˈkɒn.ʃəs/ us /-ˈkɑːn-/ › relating to this part of your mind: subconscious thoughts/fears Such memories exist only on/at the subconscious level. Our subconscious mind registers things which our conscious mind is not aware of. Cambridge Dictionaries
But Merriam Webster seems to give subconscious only as an adjective:
subconscious adjective sub·con·scious \ˌsəb-ˈkän(t)-shəs, ˈsəb-\
: existing in the part of the mind that a person is not aware of : existing in the mind but not consciously known or felt
and reserves subconsciousness as the noun.
Here's an account of Freud's uses of his original German version:
As for the term “subconscious,” Freud used it interchangeably with “unconscious” at the outset. The words are similarly close but not identical in German (subconscious is das Unterbewusste; unconscious is das Unbewusste). - Harvard Health
You can see that, in this quote, subconscious is used as a noun, translating the two German nouns.
So, going on the limited sample of MW and Cambridge (note the [uk] in the latter), is the use of the word as a noun limited to the UK? (But Harvard is on the other side of the great divide).
My feeling is that the subconscious is a Freudian reference, and therefore more closely related to psychoanalysis, whereas subconsciousness is a more general, lay use of the idea. Although the Harvard article implies we'd be better of with unconscious?
Both sentences are correct.
That comes from my subconsciousness.
That comes from my subconscious.
Clearly subconsciousness is only a noun. Subconscious is a noun and an adjective, too. ***
I (non-native-speaker) never knew before thinking about and "googling" in regard of your question, that the latter can be both, so I would have understood subconscious, as scottb ** told in his comment or to be a mistake of the writer. If someone implies a additional word it might not be the one you had in mind, e.g.
That comes from my subconscious desires.
gives an additional meaning, when the intended word would have been mind.
So you might prefer subconsciousness to subconscious when you use it as a noun, as it is not to misunderstand and therefore gives less room for interpretation.
** @EdwinAshworth: the second sentence is more informal. More than most languages, English allows for implicit subjects, particularly in informal use, eg. "Grand and State is next" for "[The station at] Grand and State is next." The second sentence also has an implied subject: "That comes from my subconscious [mind]." - scottb
*** First hit on google for "subconscious":
sub·con·scious
səbˈkänSHəs
adjective
1. of or concerning the part of the mind of which one is not fully aware but which influences one's actions and feelings. "my subconscious fear" synonyms: unconscious, latent, suppressed, repressed, subliminal, dormant, underlying, innermost; informalbottled up "subconscious desires"
noun
1. the subconscious part of the mind (not in technical use in psychoanalysis, where unconscious is preferred). synonyms: (unconscious) mind, imagination, inner(most) self, psyche "the creative powers of the subconscious"