"The Senate, whenever two-thirds of its number shall deem it necessary, shall propose amendments to this Constitution"
No mention is made of the full Senate (unless this has been replaced by an amendment) and bearing in mind how most legislatures work, I don't think that one is implied either.
Well, the interpretation that "its number" means "the whole number of Senators" was accepted in this thread.
I tend to disagree with Emsworth's supposition here, since it is clearly defined in the Constitution that the Senate has power to create rules governing its own assembly, and by right this means they have the power to choose how to define what two-thirds of their number means in real terms.
The only place in the OSPR where Abstaining is treated as a Nay is clearly marked in Article 5, Section 3, Clause 3, which states:
"3. Sections 1 and 2 of this Article shall apply in full to voting on a Veto Override, with this exception:
For the purposes of a Veto Override only, any Senator who Abstains from voting shall be counted as a vote Against the legislation under consideration."
The implication here is that only under the circumstances of a Veto Override may an Abstention be counted as a Nay against legislation. Otherwise, Abstentions do not count towards the overall "number" of Senators.
I have proposed (quite a while ago) an amendment to the OSPR which could take care of some of the confusion, if worded properly.
In this post, I laid out the concept of a "quorum requirement" and the addition of the vote "Present" to be counted against said requirement. "Abstain" would not count against the requirement or against anything for that matter.
If the amendment were so worded as to imply that the vote of "Present" would count towards the Senate's "number", then a vote of Present would count against an Amendment or Veto Override (where the "two-thirds of its number" clause is made) and not towards a simple majority vote. However, if one wanted to include it for certain types of votes and not for others, clauses could added stating such, i.e. if the Senate did not want "Present" votes to count towards amendment voting.
Just a little food for thought.