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Exhibit 31.2

# CERTIFICATION

I, Tom Manley, Senior Vice President,
Finance & Administration and Chief Financial Officer (Principal Financial Officer and
Chief Accounting Officer) certify that:

|1. |  | I
have reviewed this quarterly report on Form 10-Q of Cognos Incorporated;
|-- |  | --------------------------------------------------------------------------

|2. |  | Based
on my knowledge, this report does not contain any untrue statement of a
          material fact or omit to state a material fact necessary to make the statements
          made, in light of the circumstances under which such statements were made, not
          misleading with respect to the period covered by this report;
|-- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|3. |  | Based
on my knowledge, the financial statements, and other financial information
          included in this report, fairly present in all material respects the financial
          condition, results of operations and cash flows of the registrant as of, and
          for, the periods presented in this report;
|-- |  | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|4. |  | The
registrants other certifying officer and I are responsible for
          establishing and maintaining disclosure controls and procedures (as defined in
          Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over financial
          reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the
          registrant and have:
|-- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | a) |  | Designed
such disclosure controls and procedures, or caused such disclosure
                    controls and procedures to be designed under our supervision, to
ensure that                     material information relating to the registrant,
including its consolidated                     subsidiaries, is made known to us by
others within those entities, particularly                     during the period in which
this report is being prepared;
| | -- |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | b) |  | Designed
such internal control over financial reporting, or caused such internal
                    control over financial reporting to be designed under our
supervision, to                     provide reasonable assurance regarding the
reliability of financial reporting                     and the preparation of financial
statements for external purposes in accordance                     with generally
accepted accounting principles;
| | -- |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | c) |  | Evaluated
the effectiveness of the registrants disclosure controls and
                    procedures and presented in this report our conclusions about the
effectiveness                     of the disclosure controls and procedures, as of the
end of the period covered                     by this report based on such evaluation;
and
| | -- |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | d) |  | Disclosed
in this report any change in the registrants internal control
                    over financial reporting that occurred during the registrants
most recent                     fiscal quarter that has materially affected, or is
reasonably likely to                     materially affect, the registrants
internal control over financial                     reporting; and
| | -- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|5. |  | The
registrants other certifying officer and I have disclosed, based on           our
most recent evaluation of internal control over financial reporting, to the
          registrants auditors and the audit committee of the registrants
          board of directors (or persons performing the equivalent functions):
|-- |  | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | a) |  | All
significant deficiencies and material weaknesses in the design or operation
                    of internal control over financial reporting which are reasonably
likely to                     adversely affect the registrants ability to record,
process, summarize,                     and report financial information; and
| | -- |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | b) |  | Any
fraud, whether or not material, that involves management or other employees
                    who have a significant role in the registrants internal control
over                     financial reporting.
| | -- |  | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|October 4, 2005 |  | /s/ Tom Manley                                                                                                                                 
|--------------- |  | -----------------------------------------------------------------------------------------------------------------------------------------------
|                |  |                                                                                                                                                
|Date            |  | Tom Manley,Senior Vice President,
Finance & Administrationand
Chief Financial Officer(Principal Financial
Officer and Chief Accounting Officer)

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