Intelligent people can have difficulty understanding what a doctoral thesis is really like. If you are trying to solve the problem of the universe and everything, a doctorate for instance may take longer than 3-4 years full time study.
It is usually more manageable to critically analyse an existing body of thought on a topic, but a thesis at this level is still not so much about the use of other people's ideas as understanding and testing them.
Doctoral research is not a licence to plagiarise. Any suggestion that your thesis is not your own work must be avoided and the implications of this for referencing sources and not using other people to do work only you can do explored and followed through. It is of the nature of a doctorate that a student is able to make their own judgments about their work and take responsibility for all the dimensions that must be capable of withstanding critical scrutiny.
If near the end of your writing you still need someone else's judgment to tell you that you have got it right, then you are not yet meeting a necessary criteria for the award. It is different at an earlier stage when developing ideas and discussing them. Ways in which legitimate feedback can be obtained at any stage include presentation of a section of your work as a conference or seminar paper, and verbal discussion of ideas. These enable you to go back to refine a document which remains unambiguously your own.
It is a hard and lonely business taking the time necessary to get to a place of confidence that your thesis has a defensible coherent argument which says what you want it to say. Don't forget to take family and friends with you on the journey.
Ten common mistakes in thesis writing:
1) Being confused between a research topic and a research thesis.
2) Amassing descriptions and lists of points without developing an argument which makes sense of it all and ties it together in an integrated narrative.
3) Drawing conclusions which do not follow from the evidence presented.
4) Quoting available sources from secondary literature without checking the original and failing to assess the viewpoint and authority of sources quoted. Quoting people on issues which are marginal to a person's real area of expertise. Failure to consult multiple sources.
5) Quoting authorities on issues about which the research student must be able to form their own judgment. Students often accept things at face value it is actually the purpose of their research to test.
6) Including stuff which may have been of interest or an exciting personal discovery, yet at the end of the day is not actually part of the thesis argument. Most of us find it hard to accept that a thesis is not a compilation of everything we have done and discovered but a disciplined extract from our research arguing a thesis which can be tested. If it is not falsifiable, it is not a thesis.
7) Failure to develop a critical framework by consulting parallel studies and placing the type of argument used in the context of how others have done similar types of research project. Lack of evidence of a literature survey which shows mastery of these issues.
8) Failure to keep to word length. This is inexcusable.
9) Inadequate bibliographical searches and failure to discern the most appropriate monographs, journal articles and worldwide web resources.
10) Expecting their supervisor and others to do work on their research and writing that they have not been prepared to do themselves.