1
general::
verb VERB + SPECULATE can/could only | decline to, refuse to | be fascinating to, be interesting to, be tempting to | be idle to It is idle to speculate what the consequences would have been. | be difficult to PREP. about There was no point speculating about the possibility of them getting back together. | as to We can only speculate as to this man's identity. | on/upon He refused to speculate on her reasons for leaving.
Oxford Collocations Dictionary