In Advaita Vedanta Vedantic (or higher) reasoning is distinguished from independent reasoning or speculation, which invariably is in conflict with that of other individuals and schools of thought – ‘Speculation is unbridled… It is impossible to expect finality from it, for men’s minds are diversely inclined’ (SBh 2-1-11). The former, higher reasoning, is, or must be, in agreement with scripture (Upanishads, etc. called shruti) and is never in conflict with universal experience. There is some syllogistic deduction (‘there is fire on that mountain for we see smoke there’), but it is not prominent in AV.
‘For the truth relating to this Reality conducive to final release is too deep even for a conjuncture without revelation (SBh 2-1-11). Here ‘revelation’ means the ‘deep intuitions arrived at by the sages of old (rishis)’ and compiled in three main bodies of works (chiefly the Upanishads), so you can disregard that word and substitute ‘self-realization’ for it.
But even scriptures are not sufficient to get at the truth: a prepared, mature mind is a requisite, which usually takes years if not lifetimes. After that long preparation, preferably with the help of a qualified teacher, a final intuition (anubhava or brahmavidya) may occur. I won’t talk about the method or methods used or about the qualifications of the student, not a small matter.