A shrine is constructed for the purpose of honor, and honoring, Puja - which is accomplished in the practice or training in Yoga, respectively (or a combination of both training and practice). The purposes of training and practice are too great to easily number, and depends on the necessity and purpose of the Yogi. Necessarily, the purpose of training and practice dictate the type of shrine used: the shrine is a type of tool.
Shrines are divided into those which are constructed and those which are not. Of those which are not constructed, there are those which are made by human effort but not for the purpose of a shrine (such as a kitchen counter, or various furniture, or a wall or corner of a building, etc.) and those which are purely natural. The choice in shrine depends on its purpose.
Shrines may be grouped into Temples or Ashrams, or kept single. They may be housed and sheltered from the practice and training, or opened to the elements of their natural (or artificial) environment and the effects of the practice and training. When they are not housed or sheltered, the shrine extends through the entire place itself where the practice and training is undertaken, including the pilgrim's trail to the shrine itself. Whether the shrine is sheltered or housed, or not, depends on the purpose of the shrine.
A Temple is an (unsheltered) place for stabling multiple kinds of shrines, an Ashram may have multiple shrines, or only one, but all of the same kind - and therefore typically sheltered. Shelter is almost always provided by walls, floor and roof - but this is merely convention, due to the successful utility of a building in providing shelter. The architecture required for this sheltering requires special training. Within the Ashram, the shrine is most typically unsheltered: it is nevertheless located within every room, with one room in particular made the "puja room." Each room, and/or part of each room, is suitable for the activities required by different shrine(s), whether that is the living room, kitchen, bedroom, entry, or hallway, or the northern wall, or the southwestern corner, etc. - however the entire activities of the Ashram, both within and without the walls of the Ashram, are components of the training and practice. Again, the architecture of Ashrams requires special training.
Examples. The practice and training required for Kama necessarily requires a shrine which is not constructed, and exposed to the effects of training and practice - as these are the sacrifice required in the Puja. The practice and training required for Artha necessarily requires a constructed shrine, and one that is sheltered from the activities of practice and training, for the Artha is the benefit of the sacrifice sought. Dharmic practice and training will also be best accomplished through an unsheltered shrine which is constructed for that purpose. Brahmanic practice and training precludes the use of shrines, which are counteractive to their purpose: here, honor itself is obtained only indirectly, through abstracted intermediation, such Indra, or another Deva, or any other being, for example.
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